rip6: Fix a lock order reversal in rip6_bind()

See also commit 71a1539e3783.

Reported by: syzbot+9b461b6a07a83cc10daa@syzkaller.appspotmail.com
Reported by: syzbot+b6ce0aec16f5fdab3282@syzkaller.appspotmail.com
Reviewed by: glebius
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35472