xfrm: don't call xfrm_policy_cache_flush under xfrm_state_lock

I might be wrong but it doesn't look like xfrm_state_lock is required
for xfrm_policy_cache_flush and calling it under this lock triggers both
"sleeping function called from invalid context" and "possible circular
locking dependency detected" warnings on flush.

Fixes: ec30d78c xfrm: add xdst pcpu cache
Signed-off-by: Artem Savkov <asavkov@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>