netfilter: nf_tables: add optional user data area to rules

This allows us to store user comment strings, but it could be also
used to store any kind of information that the user application needs
to link to the rule.

Scratch 8 bits for the new ulen field that indicates the length the
user data area. 4 bits from the handle (so it's 42 bits long, according
to Patrick, it would last 139 years with 1000 new rules per second)
and 4 bits from dlen (so the expression data area is 4K, which seems
sufficient by now even considering the compatibility layer).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>