netfilter: Fix potential crash in nft_hash walker

When we get back an EAGAIN from rhashtable_walk_next we were
treating it as a valid object which obviously doesn't work too
well.

Luckily this is hard to trigger so it seems nobody has run into
it yet.

This patch fixes it by redoing the next call when we get an EAGAIN.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>