netfilter: nf_tables: simplify set dump through netlink

This patch uses the cb->data pointer that allows us to store the
context when dumping the set list. Thus, we don't need to parse the
original netlink message containing the dump request for each recvmsg()
call when dumping the set list. The different function flavours
depending on the dump criteria has been also merged into one single
generic function. This saves us ~100 lines of code.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>