netfilter: ctnetlink: Support L3 protocol-filter on flush

The same connection mark can be set on flows belonging to different
address families. This commit adds support for filtering on the L3
protocol when flushing connection track entries. If no protocol is
specified, then all L3 protocols match.

In order to avoid code duplication and a redundant check, the protocol
comparison in ctnetlink_dump_table() has been removed. Instead, a filter
is created if the GET-message triggering the dump contains an address
family. ctnetlink_filter_match() is then used to compare the L3
protocols.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>