scsi: sd: use mempool for discard special page

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>