netfilter: nft_numgen: add map lookups for numgen statements

This patch includes a new attribute in the numgen structure to allow
the lookup of an element based on the number generator as a key.

For this purpose, different ops have been included to extend the
current numgen inc functions.

Currently, only supported for numgen incremental operations, but
it will be supported for random in a follow-up patch.

Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>