netfilter: nf_tables: GC transaction race with netns dismantle

Use maybe_get_net() since GC workqueue might race with netns exit path.

Fixes: 5f68718b

 ("netfilter: nf_tables: GC transaction API to avoid race with control plane")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>