meta: permit meta nfproto ip in ip family

meta: permit meta nfproto ip in ip family

works:
add rule ip filter input ct original saddr 1.2.3.4
(family ctx init initialises network base to proto_ip).

fails to parse 1.2.3.4 address:
add rule ip filter input meta nfproto ipv4 ct original saddr 1.2.3.4

... because meta_expr_pctx_update() won't find a dependency
from "ip" to "ip" and then overwrites the correct base with proto_unknown.

"meta nfproto ip" is useless in the ip family, as it will always match,
i.e.  a better (but more compliated) fix would be to remove the statement
during evaluation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>