netfilter: x_tables: don't return garbage pointer on modprobe failure

request_module may return a positive error result from modprobe,
if we cast this to ERR_PTR this returns a garbage result (it passes
IS_ERR checks).

Fix it by ignoring modprobe return values entirely, just retry the
table lookup instead.

Reported-by:  <syzbot+980925dbfbc7f93bc2ef@syzkaller.appspotmail.com>
Fixes: 03d13b68 ("netfilter: xtables: add and use xt_request_find_table_lock")
Fixes: 20651cef ("netfilter: x_tables: unbreak module auto loading")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>