loop: don't require ->write_iter for writable files in loop_configure

Block devices can be opened read-write even if they can't be written to
for historic reasons.  Remove the check requiring file->f_op->write_iter
when the block devices was opened in loop_configure. The call to
loop_check_backing_file just below ensures the ->write_iter is present
for backing files opened for writing, which is the only check that is
actually needed.

Fixes: f5c84ef ("loop: Add sanity check for read/write_iter")
Reported-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250520135420.1177312-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

drivers/block/loop.c

@@ -979,9 +979,6 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	if (!file)
 		return -EBADF;
 
-	if ((mode & BLK_OPEN_WRITE) && !file->f_op->write_iter)
-		return -EINVAL;
-
 	error = loop_check_backing_file(file);
 	if (error)
 		return error;