uml: use of a public MAC is a warning, not an error

Downgrade one of the MAC validity checks.  If it's one that could be possibly
assigned to a physical NIC, then nothing will break.  So, emit a warning in
this case, but keep the requested MAC.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>