nvme: core: don't hold rcu read lock in nvme_ns_chr_uring_cmd_iopoll

Now nvme_ns_chr_uring_cmd_iopoll() has switched to request based io
polling, and the associated NS is guaranteed to be live in case of
io polling, so request is guaranteed to be valid because blk-mq uses
pre-allocated request pool.

Remove the rcu read lock in nvme_ns_chr_uring_cmd_iopoll(), which
isn't needed any more after switching to request based io polling.

Fix "BUG: sleeping function called from invalid context" because
set_page_dirty_lock() from blk_rq_unmap_user() may sleep.

Fixes: 585079b ("nvme: wire up async polling for io passthrough commands")
Reported-by: Guangwu Zhang <guazhang@redhat.com>
Cc: Kanchan Joshi <joshi.k@samsung.com>
Cc: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Guangwu Zhang <guazhang@redhat.com>
Link: https://lore.kernel.org/r/20230809020440.174682-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/host/ioctl.c

@@ -786,11 +786,9 @@ int nvme_ns_chr_uring_cmd_iopoll(struct io_uring_cmd *ioucmd,
 	if (!(ioucmd->flags & IORING_URING_CMD_POLLED))
 		return 0;
 
-	rcu_read_lock();
 	req = READ_ONCE(ioucmd->cookie);
 	if (req && blk_rq_is_poll(req))
 		ret = blk_rq_poll(req, iob, poll_flags);
-	rcu_read_unlock();
 	return ret;
 }
 #ifdef CONFIG_NVME_MULTIPATH