netfilter: ctnetlink: fix mark based dump filtering regression

conntrack mark based dump filtering may falsely skip entries if a mask
is given: If the mask-based check does not filter out the entry, the
else-if check is always true and compares the mark without considering
the mask. The if/else-if logic seems wrong.

Given that the mask during filter setup is implicitly set to 0xffffffff
if not specified explicitly, the mark filtering flags seem to just
complicate things. Restore the previously used approach by always
matching against a zero mask is no filter mark is given.

Fixes: cb8aa9a3 ("netfilter: ctnetlink: add kernel side filtering for dump")
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>