logfs: initialize the number of iovecs in bio

This fixes the following crash when a LogFS file system, created on a
encrypted LVM volume, was mounted

[  526.548034] BUG: unable to handle kernel NULL pointer dereference at
[  526.550106] IP: [<ffffffff8131ecab>] memcpy+0xb/0x120
[  526.551008] PGD bd60067 PUD 1778d067 PMD 0
[  526.551783] Oops: 0000 [#1] SMP

<d>Pid: 2043, comm: mount
<d>RIP: 0010:[<ffffffff8131ecab>]  [<ffffffff8131ecab>] memcpy+0xb/0x120
Call Trace:
	kcryptd_io_read+0xdb/0x100
	crypt_map+0xfd/0x190
	__map_bio+0x48/0x150
	__split_and_process_bio+0x51b/0x630
	dm_request+0x138/0x230
	generic_make_request+0xca/0x100
	submit_bio+0x87/0x110
	sync_request+0xdd/0x120 [logfs]
	bdev_readpage+0x2e/0x70 [logfs]
	do_read_cache_page+0x82/0x180
	logfs_mount+0x2ad/0x770 [logfs]
	mount_fs+0x47/0x1c0
	vfs_kern_mount+0x72/0x110
	do_kern_mount+0x54/0x110
	do_mount+0x520/0x7f0
	sys_mount+0x90/0xe0

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=42292


Reported-by: Witold Baryluk <baryluk@smp.if.uj.edu.pl>
Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>