ofproto-dpif-xlate: Adjust generated mask for fragments.

ofproto-dpif-xlate: Adjust generated mask for fragments.

It's possible to install an OpenFlow flow that matches on udp source and
destination ports without matching on fragments.  If the subtable where
such flow stays is visited during translation of a later fragment, the
generated mask will have incorrect prerequisited for the datapath and it
would be revalidated away at the first chance.

This commit fixes it by adjusting the mask for later fragments after
translation.

Other prerequisites of the mask are also prerequisites in OpenFlow, but
not the ip fragment bit, that's why we need a special case here.

For completeness, this commits also fixes a related problem in bfd,
where we check the udp destination port without checking if the frame is
an ip fragment.  It's not really necessary to address this separately,
given the adjustment that we perform.

VMware-BZ: #1651589
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Jarno Rajahalme <jarno@ovn.org>