When sending a routing message, don't allow the user to set the
RTF_RNH_LOCKED flag in rtm_flags, since this flag is used only
internally.

Reported by: syzbot+65c676f5248a13753ea0@syzkaller.appspotmail.com
Reviewed by: ae@
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19898