block: loop: restore backing file state on loop_configure() failure

If queue_limits_commit_update() fails after loop_assign_backing_file(), restore the backing file GFP mask, clear lo_backing_file and lo_device, and re-enable uevents before unlocking.

Scope the rollback to a dedicated out_restore_backing label reached only from errors after the backing file is modified in this call. The common out_unlock path is also used for early validation failures such as -EBUSY when lo->lo_state != Lo_unbound; in that case the device may already have a valid backing file that must not be cleared.

---
Changes since v1:
- Drop the brd xa_destroy patch: xa_init() does not allocate; there is nothing to destroy on the blk_alloc_disk() failure path.
- Fix loop rollback scope: do not restore from the common out_unlock path. That label is also reached for early failures such as -EBUSY when the device is already bound, where lo_backing_file belongs to the existing configuration. Use a dedicated out_restore_backing label only for errors after loop_assign_backing_file().

 drivers/block/loop.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

index 6f12976035b0..aec7fbad1903 100644
@@ -1085,7 +1085,7 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 	/* No need to freeze the queue as the device isn't bound yet. */
 	error = queue_limits_commit_update(lo->lo_queue, &lim);
 	if (error)
-		goto out_unlock;
+		goto out_restore_backing;
 
 	/*
 	 * We might switch to direct I/O mode for the loop device, write back

@@ -1122,6 +1122,13 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 
 	return 0;
 
+out_restore_backing:
+	mapping_set_gfp_mask(lo->lo_backing_file->f_mapping,
+			     lo->old_gfp_mask);
+	lo->lo_backing_file = NULL;
+	lo->lo_device = NULL;
+	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
+
 out_unlock:
 	loop_global_unlock(lo, is_loop);
 out_bdev:

2.43.0