xfrm: state: make xfrm_state_lookup_byaddr lockless

This appears to be an oversight back when the state lookup
was converted to RCU, I see no reason why we need to hold the
state lock here.

__xfrm_state_lookup_byaddr already uses xfrm_state_hold_rcu
helper to obtain a reference, so just replace the state
lock with rcu.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>