nvmet: fix race in nvmet_bio_done() leading to NULL pointer dereference

nvmet: fix race in nvmet_bio_done() leading to NULL pointer dereference

There is a race condition in nvmet_bio_done() that can cause a NULL
pointer dereference in blk_cgroup_bio_start():

1. nvmet_bio_done() is called when a bio completes
2. nvmet_req_complete() is called, which invokes req->ops->queue_response(req)
3. The queue_response callback can re-queue and re-submit the same request
4. The re-submission reuses the same inline_bio from nvmet_req
5. Meanwhile, nvmet_req_bio_put() (called after nvmet_req_complete)
   invokes bio_uninit() for inline_bio, which sets bio->bi_blkg to NULL
6. The re-submitted bio enters submit_bio_noacct_nocheck()
7. blk_cgroup_bio_start() dereferences bio->bi_blkg, causing a crash:

  BUG: kernel NULL pointer dereference, address: 0000000000000028
  #PF: supervisor read access in kernel mode
  RIP: 0010:blk_cgroup_bio_start+0x10/0xd0
  Call Trace:
   submit_bio_noacct_nocheck+0x44/0x250
   nvmet_bdev_execute_rw+0x254/0x370 [nvmet]
   process_one_work+0x193/0x3c0
   worker_thread+0x281/0x3a0

Fix this by reordering nvmet_bio_done() to call nvmet_req_bio_put()
BEFORE nvmet_req_complete(). This ensures the bio is cleaned up before
the request can be re-submitted, preventing the race condition.

Fixes: 190f4c2c863a ("nvmet: fix memory leak of bio integrity")
Cc: Dmitry Bogdanov <d.bogdanov@yadro.com>
Cc: stable@vger.kernel.org
Cc: Guangwu Zhang <guazhang@redhat.com>
Link: http://www.mail-archive.com/debian-kernel@lists.debian.org/msg146238.html
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>