[XFRM]: fix incorrect xfrm_state_afinfo_lock use

xfrm_state_afinfo_lock can be read-locked from bh context, so take it
in a bh-safe manner in xfrm_state_register_afinfo() and
xfrm_state_unregister_afinfo(). Found by the lock validator.

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