i2c: xiic: Prevent concurrent running of the IRQ handler and __xiic_start_xfer()

Prior to commit e6c9a037 ("i2c: xiic: Remove the disabling of
interrupts") IRQs where disabled when the initial __xiic_start_xfer() was
called. After the commit the interrupt is enabled while the function is
running, this means it is possible for the interrupt to be triggered while
the function is still running. When this happens the internal data
structures get corrupted and undefined behavior can occur like the
following crash:

	Internal error: Oops: 17 [#1] PREEMPT SMP ARM
	Modules linked in:
	CPU: 0 PID: 2040 Comm: i2cdetect Not tainted 4.0.0-02856-g047a308 #10956
	Hardware name: Xilinx Zynq Platform
	task: ee0c9500 ti: e99a2000 task.ti: e99a2000
	PC is at __xiic_start_xfer+0x6c4/0x7c8
	LR is at __xiic_start_xfer+0x690/0x7c8
	pc : [<c02bbffc>]    lr : [<c02bbfc8>]    psr: 800f0013
	sp : e99a3da8  ip : 00000000  fp : 00000000
	r10: 00000001  r9 : 600f0013  r8 : f0180000
	r7 : f0180000  r6 : c064e444  r5 : 00000017  r4 : ee031010
	r3 : 00000000  r2 : 00000000  r1 : 600f0013  r0 : 0000000f
	Flags: Nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
	Control: 18c5387d  Table: 29a5404a  DAC: 00000015
	Process i2cdetect (pid: 2040, stack limit = 0xe99a2210)
	Stack: (0xe99a3da8 to 0xe99a4000)
	3da0:                   ee031010 00000000 00000001 ee031020 ee031224 c02bc5ec
	3dc0: ee34c604 00000000 ee0c9500 e99a3dcc e99a3dd0 e99a3dd0 e99a3dd8 c069f0e8
	3de0: 00000000 ee031020 c064e100 ffff90bb e99a3e48 c02b6590 ee031020 00000001
	3e00: e99a3e48 ee031020 00000000 e99a3e63 00000001 c02b6ec4 00000000 00000000
	3e20: 00000000 c02b7320 e99a3ef0 00000000 00000000 e99e3df0 00000000 00000000
	3e40: 00000103 2814575f 0000003e c00a0000 e99a3e85 0001003e ee0c0000 e99a3e63
	3e60: eefd3578 c064e61c ee0c9500 c0041e04 0000056c e9a56db8 00006e5a b6f5c000
	3e80: ee0c9548 eefd0040 00000001 eefd3540 ee0c9500 eefd39a0 c064b540 ee0c9500
	3ea0: 00000000 ee92b000 00000000 bef4862c ee34c600 e99ecdc0 00000720 00000003
	3ec0: e99a2000 00000000 00000000 c02b8b30 00000000 00000000 00000000 e99a3f24
	3ee0: b6e80000 00000000 00000000 c04257e8 00000000 e99a3f24 c02b8f08 00000703
	3f00: 00000003 c02116bc ee935300 00000000 bef4862c ee34c600 e99ecdc0 c02b91f0
	3f20: e99ecdc0 00000720 bef4862c eeb725f8 e99ecdc0 c00c9e2c 00000003 00000003
	3f40: ee248dc0 00000000 ee248dc8 00000002 eeb7c1a8 00000000 00000000 c00bb360
	3f60: 00000000 00000000 00000003 ee248dc0 bef4862c e99ecdc0 e99ecdc0 00000720
	3f80: 00000003 e99a2000 00000000 c00c9f68 00000000 00000000 b6f22000 00000036
	3fa0: c000dfa4 c000de20 00000000 00000000 00000003 00000720 bef4862c bef4862c
	3fc0: 00000000 00000000 b6f22000 00000036 00000000 00000000 b6f60000 00000000
	3fe0: 00013040 bef48614 00008cab b6ecdbe6 400f0030 00000003 2f7fd821 2f7fdc21
	[<c02bbffc>] (__xiic_start_xfer) from [<c02bc5ec>] (xiic_xfer+0x94/0x168)
	[<c02bc5ec>] (xiic_xfer) from [<c02b6590>] (__i2c_transfer+0x4c/0x7c)
	[<c02b6590>] (__i2c_transfer) from [<c02b6ec4>] (i2c_transfer+0x9c/0xc4)
	[<c02b6ec4>] (i2c_transfer) from [<c02b7320>] (i2c_smbus_xfer+0x3a0/0x4ec)
	[<c02b7320>] (i2c_smbus_xfer) from [<c02b8b30>] (i2cdev_ioctl_smbus+0xb0/0x214)
	[<c02b8b30>] (i2cdev_ioctl_smbus) from [<c02b91f0>] (i2cdev_ioctl+0xa0/0x1d4)
	[<c02b91f0>] (i2cdev_ioctl) from [<c00c9e2c>] (do_vfs_ioctl+0x4b0/0x5b8)
	[<c00c9e2c>] (do_vfs_ioctl) from [<c00c9f68>] (SyS_ioctl+0x34/0x5c)
	[<c00c9f68>] (SyS_ioctl) from [<c000de20>] (ret_fast_syscall+0x0/0x34)
	Code: e283300c e5843210 eafffe64 e5943210 (e1d320b4)

The issue can easily be reproduced by performing I2C access under high
system load or IO load.

To fix the issue protect the invocation to __xiic_start_xfer() form
xiic_start_xfer() with the same lock that is used to protect the interrupt
handler.

Fixes: e6c9a037 ("i2c: xiic: Remove the disabling of interrupts")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>