btrfs: don't get an EINTR during drop_snapshot for reloc

BugLink: https://bugs.launchpad.net/bugs/1915195

commit 18d3bff upstream.

This was partially fixed by f3e3d9c ("btrfs: avoid possible signal
interruption of btrfs_drop_snapshot() on relocation tree"), however it
missed a spot when we restart a trans handle because we need to end the
transaction.  The fix is the same, simply use btrfs_join_transaction()
instead of btrfs_start_transaction() when deleting reloc roots.

Fixes: f3e3d9c ("btrfs: avoid possible signal interruption of btrfs_drop_snapshot() on relocation tree")
CC: [email protected] # 5.4+
Signed-off-by: Josef Bacik <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Kamal Mostafa <[email protected]>
Signed-off-by: William Breathitt Gray <[email protected]>

Author: josefbacik

fs/btrfs/extent-tree.c

@@ -5542,7 +5542,15 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 				goto out_free;
 			}
 
-			trans = btrfs_start_transaction(tree_root, 0);
+		       /*
+			* Use join to avoid potential EINTR from transaction
+			* start. See wait_reserve_ticket and the whole
+			* reservation callchain.
+			*/
+			if (for_reloc)
+				trans = btrfs_join_transaction(tree_root);
+			else
+				trans = btrfs_start_transaction(tree_root, 0);
 			if (IS_ERR(trans)) {
 				err = PTR_ERR(trans);
 				goto out_free;