aoe: update and specify AoE address guards and error messages

In general, specific is better when it comes to messages about AoE usage
problems.  Also, explicit checks for the AoE broadcast addresses are
added.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>