aoe: properly initialise the request_queue's backing_dev_info

aoe: properly initialise the request_queue's backing_dev_info

AOE forgot to initialise its queue's backing_dev_info, so kernels crash.
(http://bugzilla.kernel.org/show_bug.cgi?id=9482)

Fix that and consoldate aoeblk_gdalloc()'s error handling.

Thanks be to Jon for reporting and testing.

Cc: "Ed L. Cashin" <ecashin@coraid.com>
Cc: <stable@kernel.org>
Cc: "Jon Nelson" <jnelson@jamponi.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>