netfilter: x_tables: unlock on error in xt_find_table_lock()

According to my static checker we should unlock here before the return.
That seems reasonable to me as well.

Fixes" b9e69e12 ("netfilter: xtables: don't hook tables by default")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>