net: Fix RCU splat in af_key

Hit the following splat testing VRF change for ipsec:

[  113.475692] ===============================
[  113.476194] [ INFO: suspicious RCU usage. ]
[  113.476667] 4.2.0-rc6-1+deb7u2+clUNRELEASED #3.2.65-1+deb7u2+clUNRELEASED Not tainted
[  113.477545] -------------------------------
[  113.478013] /work/monster-14/dsa/kernel.git/include/linux/rcupdate.h:568 Illegal context switch in RCU read-side critical section!
[  113.479288]
[  113.479288] other info that might help us debug this:
[  113.479288]
[  113.480207]
[  113.480207] rcu_scheduler_active = 1, debug_locks = 1
[  113.480931] 2 locks held by setkey/6829:
[  113.481371]  #0:  (&net->xfrm.xfrm_cfg_mutex){+.+.+.}, at: [<ffffffff814e9887>] pfkey_sendmsg+0xfb/0x213
[  113.482509]  #1:  (rcu_read_lock){......}, at: [<ffffffff814e767f>] rcu_read_lock+0x0/0x6e
[  113.483509]
[  113.483509] stack backtrace:
[  113.484041] CPU: 0 PID: 6829 Comm: setkey Not tainted 4.2.0-rc6-1+deb7u2+clUNRELEASED #3.2.65-1+deb7u2+clUNRELEASED
[  113.485422] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5.1-0-g8936dbb-20141113_115728-nilsson.home.kraxel.org 04/01/2014
[  113.486845]  0000000000000001 ffff88001d4c7a98 ffffffff81518af2 ffffffff81086962
[  113.487732]  ffff88001d538480 ffff88001d4c7ac8 ffffffff8107ae75 ffffffff8180a154
[  113.488628]  0000000000000b30 0000000000000000 00000000000000d0 ffff88001d4c7ad8
[  113.489525] Call Trace:
[  113.489813]  [<ffffffff81518af2>] dump_stack+0x4c/0x65
[  113.490389]  [<ffffffff81086962>] ? console_unlock+0x3d6/0x405
[  113.491039]  [<ffffffff8107ae75>] lockdep_rcu_suspicious+0xfa/0x103
[  113.491735]  [<ffffffff81064032>] rcu_preempt_sleep_check+0x45/0x47
[  113.492442]  [<ffffffff8106404d>] ___might_sleep+0x19/0x1c8
[  113.493077]  [<ffffffff81064268>] __might_sleep+0x6c/0x82
[  113.493681]  [<ffffffff81133190>] cache_alloc_debugcheck_before.isra.50+0x1d/0x24
[  113.494508]  [<ffffffff81134876>] kmem_cache_alloc+0x31/0x18f
[  113.495149]  [<ffffffff814012b5>] skb_clone+0x64/0x80
[  113.495712]  [<ffffffff814e6f71>] pfkey_broadcast_one+0x3d/0xff
[  113.496380]  [<ffffffff814e7b84>] pfkey_broadcast+0xb5/0x11e
[  113.497024]  [<ffffffff814e82d1>] pfkey_register+0x191/0x1b1
[  113.497653]  [<ffffffff814e9770>] pfkey_process+0x162/0x17e
[  113.498274]  [<ffffffff814e9895>] pfkey_sendmsg+0x109/0x213

In pfkey_sendmsg the net mutex is taken and then pfkey_broadcast takes
the RCU lock.

Since pfkey_broadcast takes the RCU lock the allocation argument is
pointless since GFP_ATOMIC must be used between the rcu_read_{,un}lock.
The one call outside of rcu can be done with GFP_KERNEL.

Fixes: 7f6b9dbd ("af_key: locking change")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>