scsi: sd: use mempool for discard special page

commit 61cce6f6eeced5ddd9cac55e807fe28b4f18c1ba upstream.

When boxes are run near (or to) OOM, we have a problem with the discard
page allocation in sd. If we fail allocating the special page, we return
busy, and it'll get retried. But since ordering is honored for dispatch
requests, we can keep retrying this same IO and failing. Behind that IO
could be requests that want to free memory, but they never get the
chance. This means you get repeated spews of traces like this:

[1201401.625972] Call Trace:
[1201401.631748]  dump_stack+0x4d/0x65
[1201401.639445]  warn_alloc+0xec/0x190
[1201401.647335]  __alloc_pages_slowpath+0xe84/0xf30
[1201401.657722]  ? get_page_from_freelist+0x11b/0xb10
[1201401.668475]  ? __alloc_pages_slowpath+0x2e/0xf30
[1201401.679054]  __alloc_pages_nodemask+0x1f9/0x210
[1201401.689424]  alloc_pages_current+0x8c/0x110
[1201401.699025]  sd_setup_write_same16_cmnd+0x51/0x150
[1201401.709987]  sd_init_command+0x49c/0xb70
[1201401.719029]  scsi_setup_cmnd+0x9c/0x160
[1201401.727877]  scsi_queue_rq+0x4d9/0x610
[1201401.736535]  blk_mq_dispatch_rq_list+0x19a/0x360
[1201401.747113]  blk_mq_sched_dispatch_requests+0xff/0x190
[1201401.758844]  __blk_mq_run_hw_queue+0x95/0xa0
[1201401.768653]  blk_mq_run_work_fn+0x2c/0x30
[1201401.777886]  process_one_work+0x14b/0x400
[1201401.787119]  worker_thread+0x4b/0x470
[1201401.795586]  kthread+0x110/0x150
[1201401.803089]  ? rescuer_thread+0x320/0x320
[1201401.812322]  ? kthread_park+0x90/0x90
[1201401.820787]  ? do_syscall_64+0x53/0x150
[1201401.829635]  ret_from_fork+0x29/0x40

Ensure that the discard page allocation has a mempool backing, so we
know we can make progress.

Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>