xfrm: Fix xfrm_policy_lock imbalance

An earlier patch accidentally replaced a write_lock_bh
with a spin_unlock_bh. Fix this by using spin_lock_bh
instead.

Fixes: 9d0380df ("xfrm: policy: convert policy_lock to spinlock")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>