Fix two bugs with the bridge

Fix two bugs with the bridge

 - code expects memcmp() to return a signed value, our memcmp() returns 0 if
   args are equal and > 0 if not.

 - It's possible to hijack interface for static entry. If bridge recieves
   packet from interface marked as learning it will replace the bridge_rtnode
   entry for the source address even if such entry marked as static.

Submitted by:	Gleb Kurtsov <k-gleb yandex.ru>
MFC after:	3 days