btrfs: fall back to global reservation when removing subvolumes

I recently did some ENOSPC testing that involved filling the disk
while create and removing snapshots in a loop. During the test cycle,
I ran into an ENOSPC when trying to remove a snapshot, leaving the fs
stuck in ENOSPC even after a umount/mount cycle.

This patch allow subvolume removal to fall back onto the global
block reservation in order to succeed when it would have failed
otherwise.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 238a055..841044b 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -396,7 +396,7 @@
 	 * of create_snapshot().
 	 */
 	ret = btrfs_subvolume_reserve_metadata(root, &block_rsv,
-					       7, &qgroup_reserved);
+					       7, &qgroup_reserved, false);
 	if (ret)
 		return ret;
 
@@ -576,7 +576,8 @@
 	 */
 	ret = btrfs_subvolume_reserve_metadata(BTRFS_I(dir)->root,
 					&pending_snapshot->block_rsv, 7,
-					&pending_snapshot->qgroup_reserved);
+					&pending_snapshot->qgroup_reserved,
+					false);
 	if (ret)
 		goto out;
 
@@ -2174,7 +2175,7 @@
 	 * ref/backref.
 	 */
 	err = btrfs_subvolume_reserve_metadata(root, &block_rsv,
-					       5, &qgroup_reserved);
+					       5, &qgroup_reserved, true);
 	if (err)
 		goto out_up_write;