udp: fix sending of IPv4-mapped addresses

The inp_vflags field must be adjusted during the call of
in_pcbbind_setup(). This is consistent with the other places in the
code, but not elegant at all.

PR: 274009
Reported by: syzbot+81ccc423a2737ed031ac@syzkaller.appspotmail.com
Reported by: syzbot+c8e3dac881bba85bc029@syzkaller.appspotmail.com
Reviewed by: markj, rrs, rscheff
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42031