aoe: associate frames with the AoE storage target

In the driver code, "target" and aoetgt refer to a particular remote
interface on the AoE storage target.  The latter is identified by its AoE
major and minor addresses.  Commands that are being sent to an AoE storage
target {major, minor} can be sent or retransmitted to any of the remote
MAC addresses associated with the AoE storage target.

That is, frames are naturally associated with not an aoetgt (AoE major,
AoE minor, remote MAC address) but an aoedev (AoE major, AoE minor).
Making the code reflect that reality simplifies the driver, especially
when the path to a remote MAC address becomes unusable.

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>