loop: use kiocb helpers to fix lockdep warning

JIRA: https://issues.redhat.com/browse/RHEL-103981

commit c4706c5
Author: Ming Lei <ming.lei@redhat.com>
Date:   Wed Jul 16 19:48:08 2025 +0800

    loop: use kiocb helpers to fix lockdep warning

    The lockdep tool can report a circular lock dependency warning in the loop
    driver's AIO read/write path:

    ```
    [ 6540.587728] kworker/u96:5/72779 is trying to acquire lock:
    [ 6540.593856] ff110001b5968440 (sb_writers#9){.+.+}-{0:0}, at: loop_process_work+0x11a/0xf70 [loop]
    [ 6540.603786]
    [ 6540.603786] but task is already holding lock:
    [ 6540.610291] ff110001b5968440 (sb_writers#9){.+.+}-{0:0}, at: loop_process_work+0x11a/0xf70 [loop]
    [ 6540.620210]
    [ 6540.620210] other info that might help us debug this:
    [ 6540.627499]  Possible unsafe locking scenario:
    [ 6540.627499]
    [ 6540.634110]        CPU0
    [ 6540.636841]        ----
    [ 6540.639574]   lock(sb_writers#9);
    [ 6540.643281]   lock(sb_writers#9);
    [ 6540.646988]
    [ 6540.646988]  *** DEADLOCK ***
    ```

    This patch fixes the issue by using the AIO-specific helpers
    `kiocb_start_write()` and `kiocb_end_write()`. These functions are
    designed to be used with a `kiocb` and manage write sequencing
    correctly for asynchronous I/O without introducing the problematic
    lock dependency.

    The `kiocb` is already part of the `loop_cmd` struct, so this change
    also simplifies the completion function `lo_rw_aio_do_completion()` by
    using the `iocb` from the `cmd` struct directly, instead of retrieving
    the loop device from the request queue.

    Fixes: 39d86db ("loop: add file_start_write() and file_end_write()")
    Cc: Changhui Zhong <czhong@redhat.com>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Link: https://lore.kernel.org/r/20250716114808.3159657-1-ming.lei@redhat.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

drivers/block/loop.c

@@ -308,14 +308,13 @@ static void lo_complete_rq(struct request *rq)
 static void lo_rw_aio_do_completion(struct loop_cmd *cmd)
 {
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
-	struct loop_device *lo = rq->q->queuedata;
 
 	if (!atomic_dec_and_test(&cmd->ref))
 		return;
 	kfree(cmd->bvec);
 	cmd->bvec = NULL;
 	if (req_op(rq) == REQ_OP_WRITE)
-		file_end_write(lo->lo_backing_file);
+		kiocb_end_write(&cmd->iocb);
 	if (likely(!blk_should_fake_timeout(rq->q)))
 		blk_mq_complete_request(rq);
 }
@@ -391,7 +390,7 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	}
 
 	if (rw == ITER_SOURCE) {
-		file_start_write(lo->lo_backing_file);
+		kiocb_start_write(&cmd->iocb);
 		ret = file->f_op->write_iter(&cmd->iocb, &iter);
 	} else
 		ret = file->f_op->read_iter(&cmd->iocb, &iter);