Ensure that the flags indicating IPv4/IPv6 are not changed by failing
bind() calls. This would lead to inconsistent state resulting in a panic.
A fix for stable/11 was committed in
https://svnweb.freebsd.org/base?view=revision&revision=338986
An accelerated MFC is planned as discussed with emaste@.

Reported by: syzbot+2609a378d89264ff5a42@syzkaller.appspotmail.com
Obtained from: jtl@
MFC after: 1 day
Sponsored by: Netflix, Inc.