xfrm: policy: don't acquire policy lock in xfrm_spd_getinfo

It doesn't seem that important.

We now get inconsistent view of the counters, but those are stale anyway
right after we drop the lock.

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