Btrfs: cleanup reloc roots properly on error

I was hitting the BUG_ON() at the end of merge_reloc_roots() because we were
aborting the transaction at some point previously and then getting an error when
we tried to drop the reloc root.  I fixed btrfs_drop_snapshot to re-add us to
the dead roots list if we failed, but this isn't the right thing to do for reloc
roots since it uses root->root_list for it's own stuff in order to know what
needs to be cleaned up.  So fix btrfs_drop_snapshot to only do the re-add if we
aren't dropping for reloc, and handle errors from merge_reloc_root() by dropping
the reloc root we are processing since it won't be on the list of roots to
cleanup.  With this patch my reproducer no longer panics.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/extent-tree.c

@@ -7696,7 +7696,7 @@ out:
 	 * don't have it in the radix (like when we recover after a power fail
 	 * or unmount) so we don't leak memory.
 	 */
-	if (root_dropped == false)
+	if (!for_reloc && root_dropped == false)
 		btrfs_add_dead_root(root);
 	if (err)
 		btrfs_std_error(root->fs_info, err);

fs/btrfs/relocation.c

@@ -2314,8 +2314,13 @@ again:
 			BUG_ON(root->reloc_root != reloc_root);
 
 			ret = merge_reloc_root(rc, root);
-			if (ret)
+			if (ret) {
+				__update_reloc_root(reloc_root, 1);
+				free_extent_buffer(reloc_root->node);
+				free_extent_buffer(reloc_root->commit_root);
+				kfree(reloc_root);
 				goto out;
+			}
 		} else {
 			list_del_init(&reloc_root->root_list);
 		}