netfilter: nfnetlink: fix potential dead lock in nfnetlink_rcv_msg()

When type is NFNL_CB_MUTEX and -EAGAIN error occur in nfnetlink_rcv_msg(),
it does not execute nfnl_unlock(). That would trigger potential dead lock.

Fixes: 50f2db9e ("netfilter: nfnetlink: consolidate callback types")
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: Florian Westphal <fw@strlen.de>