[XFRM]: fix incorrect xfrm_policy_afinfo_lock use

[XFRM]: fix incorrect xfrm_policy_afinfo_lock use

xfrm_policy_afinfo_lock can be taken in bh context, at:

 [<c013fe1a>] lockdep_acquire_read+0x54/0x6d
 [<c0f6e024>] _read_lock+0x15/0x22
 [<c0e8fcdb>] xfrm_policy_get_afinfo+0x1a/0x3d
 [<c0e8fd10>] xfrm_decode_session+0x12/0x32
 [<c0e66094>] ip_route_me_harder+0x1c9/0x25b
 [<c0e770d3>] ip_nat_local_fn+0x94/0xad
 [<c0e2bbc8>] nf_iterate+0x2e/0x7a
 [<c0e2bc50>] nf_hook_slow+0x3c/0x9e
 [<c0e3a342>] ip_push_pending_frames+0x2de/0x3a7
 [<c0e53e19>] icmp_push_reply+0x136/0x141
 [<c0e543fb>] icmp_reply+0x118/0x1a0
 [<c0e54581>] icmp_echo+0x44/0x46
 [<c0e53fad>] icmp_rcv+0x111/0x138
 [<c0e36764>] ip_local_deliver+0x150/0x1f9
 [<c0e36be2>] ip_rcv+0x3d5/0x413
 [<c0df760f>] netif_receive_skb+0x337/0x356
 [<c0df76c3>] process_backlog+0x95/0x110
 [<c0df5fe2>] net_rx_action+0xa5/0x16d
 [<c012d8a7>] __do_softirq+0x6f/0xe6
 [<c0105ec2>] do_softirq+0x52/0xb1

this means that all write-locking of xfrm_policy_afinfo_lock must be
bh-safe. This patch fixes xfrm_policy_register_afinfo() and
xfrm_policy_unregister_afinfo().

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>