netfilter: ctnetlink: Fix regression in CTA_STATUS processing

The libnetfilter_conntrack userland library always sets IPS_CONFIRMED
when building a CTA_STATUS attribute.  If this toggles the bit from
0->1, the parser will return an error.  On Linux 4.4+ this will cause any
NFQA_EXP attribute in the packet to be ignored.  This breaks conntrackd's
userland helpers because they operate on unconfirmed connections.

Instead of returning -EBUSY if the user program asks to modify an
unchangeable bit, simply ignore the change.

Also, fix the logic so that user programs are allowed to clear
the bits that they are allowed to change.

Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>