r348494 fixes a race in udp_output(). The same race exists in
udp_output6(), therefore apply a similar patch to IPv6.

Reported by: syzbot+c5ffbc8f14294c7b0e54@syzkaller.appspotmail.com
Reviewed by: bz@, markj@
MFC after: 2 weeks
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D20936