We need not mind if users grab a late handle on a closing disk. We
probably even should not. But we have to make sure it's not a dead
one already

Let the bdev deal with a gendisk deleted under its feet. Takes the
info mutex to decide a race against backend closing.
Signed-off-by: NDaniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>