Unbreak, don't assume we always hold the NET_LOCK

Unbreak, don't assume we always hold the NET_LOCK

After net/if.c:rev 1.661 ifioctl() doesn't take the net lock any more
around SIOCGIFMEDIA and SIOCSIFMEDIA, which broke cad_ioctl().
The same was already true about SIOCGIFSFFPAGE.  Only release/reacquire
the net lock if warranted.

Input mvs@, ok miod@ mvs@ bluhm@