netfilter: nft_inner: validate mandatory meta and payload

Check for mandatory netlink attributes in payload and meta expression
when used embedded from the inner expression, otherwise NULL pointer
dereference is possible from userspace.

Fixes: a150d122 ("netfilter: nft_meta: add inner match support")
Fixes: 3a07327d ("netfilter: nft_inner: support for inner tunnel header matching")
Signed-off-by: Davide Ornaghi <d.ornaghi97@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>