xfrm: Fix xfrm_policy_gc_lock handling.

From: Alexey Dobriyan <adobriyan@gmail.com>

Based upon a lockdep trace by Simon Arlott.

xfrm_policy_kill() can be called from both BH and
non-BH contexts, so we have to grab xfrm_policy_gc_lock
with BH disabling.

Signed-off-by: David S. Miller <davem@davemloft.net>