Don't indirect user pointers directly in two 802.11s ioctls.

IEEE80211_MESH_RTCMD_ADD was invoking memcmp() to validate the
supplied address directly on the user pointer rather than first doing
a copyin() and validating the copied value.

IEEE80211_MESH_RTCMD_DELETE was passing the user pointer directly to
ieee80211_mesh_rt_del() rather than copying the user buffer into a
temporary kernel buffer.

Reviewed by: brooks, kib
Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24562