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 <[e-mail]>
Cc: [e-mail]
Cc: Guangwu Zhang <[e-mail]>
Link: http://www.mail-archive.com/[e-mail]/msg146238.html
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Keith Busch <[e-mail]>

drivers/nvme/target/io-cmd-bdev.c

···
 static void nvmet_bio_done(struct bio *bio)
 {
 	struct nvmet_req *req = bio->bi_private;
+	blk_status_t blk_status = bio->bi_status;
 
-	nvmet_req_complete(req, blk_to_nvme_status(req, bio->bi_status));
 	nvmet_req_bio_put(req, bio);
+	nvmet_req_complete(req, blk_to_nvme_status(req, blk_status));
 }
 
 #ifdef CONFIG_BLK_DEV_INTEGRITY