netfilter: nfnetlink: fix missing rcu_read_unlock in nfnetlink_rcv_msg

Bug added in commit 6b75e3e8 (netfilter: nfnetlink: add RCU in
nfnetlink_rcv_msg())

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>