loop: don't set QUEUE_FLAG_NOMERGES

QUEUE_FLAG_NOMERGES isn't really a driver interface, but a user tunable.
There also isn't any good reason to set it in the loop driver.

The original commit adding it (5b5e20f "block: loop: set
QUEUE_FLAG_NOMERGES for request queue of loop") claims that "It doesn't
make sense to enable merge because the I/O submitted to backing file is
handled page by page."  which of course isn't true for multi-page bvec
now, and it never has been for direct I/O, for which commit 40326d8
("block/loop: allow request merge for directio mode") alredy disabled
the nomerges flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240627124926.512662-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

drivers/block/loop.c

@@ -211,13 +211,10 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
 	if (lo->lo_state == Lo_bound)
 		blk_mq_freeze_queue(lo->lo_queue);
 	lo->use_dio = use_dio;
-	if (use_dio) {
-		blk_queue_flag_clear(QUEUE_FLAG_NOMERGES, lo->lo_queue);
+	if (use_dio)
 		lo->lo_flags |= LO_FLAGS_DIRECT_IO;
-	} else {
-		blk_queue_flag_set(QUEUE_FLAG_NOMERGES, lo->lo_queue);
+	else
 		lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
-	}
 	if (lo->lo_state == Lo_bound)
 		blk_mq_unfreeze_queue(lo->lo_queue);
 }
@@ -2030,14 +2027,6 @@ static int loop_add(int i)
 	}
 	lo->lo_queue = lo->lo_disk->queue;
 
-	/*
-	 * By default, we do buffer IO, so it doesn't make sense to enable
-	 * merge because the I/O submitted to backing file is handled page by
-	 * page. For directio mode, merge does help to dispatch bigger request
-	 * to underlayer disk. We will enable merge once directio is enabled.
-	 */
-	blk_queue_flag_set(QUEUE_FLAG_NOMERGES, lo->lo_queue);
-
 	/*
 	 * Disable partition scanning by default. The in-kernel partition
 	 * scanning can be requested individually per-device during its