netfilter: nf_nat: remove obsolete rcu_read_unlock call

hlist walk in find_appropriate_src() is not protected anymore by rcu_read_lock(),
so rcu_read_unlock() is unnecessary if in_range() matches.

This bug was added in (c7232c9 netfilter: add protocol independent NAT core).

Signed-off-by: Ulrich Weber <ulrich.weber@sophos.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>