netfilter: nf_tables: remove WARN and add NLA_STRING upper limits

This WARN can trigger because some of the names fed to the module
autoload function can be of arbitrary length.

Remove the WARN and add limits for all NLA_STRING attributes.

Reported-by:  <syzbot+0e63ae76d117ae1c3a01@syzkaller.appspotmail.com>
Fixes: 452238e8 ("netfilter: nf_tables: add and use helper for module autoload")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>