This patch fixes a lost request discovered during IO + hot removal.

The driver's pci removal deletes gendisks prior to shutting down the
controller to allow dirty data to sync. Dirty data can not be synced on
a surprise removal, though, and would potentially block indefinitely.

The driver previously had marked the queue as dying in this scenario
to prevent new requests from attempting, however it will still block
for requests that already entered the queue. This patch fixes this by
quiescing IO first, then aborting the requeued requests before deleting
disks.
Reported-by: Sujith Pandel <sujith_pandel@dell.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Tested-by: Sujith Pandel <sujith_pandel@dell.com>
Signed-off-by: Jens Axboe <axboe@fb.com>