netfilter: nf_tables: adjust lockdep assertions handling

It's needed to check the return value of lockdep_commit_lock_is_held(),
otherwise there's no point in this assertion as it doesn't print any
debug information on itself.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.

Fixes: b04df3da ("netfilter: nf_tables: do not defer rule destruction via call_rcu")
Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>