btrfs: cleanup error handling in prepare_to_merge

This probably can't happen even with a corrupt file system, because we
would have failed much earlier on than here.  However there's no reason
we can't just check and bail out as appropriate, so do that and convert
the correctness BUG_ON() to an ASSERT().
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -1885,8 +1885,18 @@ int prepare_to_merge(struct reloc_control *rc, int err)
 
 		root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset,
 				false);
-		BUG_ON(IS_ERR(root));
-		BUG_ON(root->reloc_root != reloc_root);
+		if (IS_ERR(root)) {
+			/*
+			 * Even if we have an error we need this reloc root
+			 * back on our list so we can clean up properly.
+			 */
+			list_add(&reloc_root->root_list, &reloc_roots);
+			btrfs_abort_transaction(trans, (int)PTR_ERR(root));
+			if (!err)
+				err = PTR_ERR(root);
+			break;
+		}
+		ASSERT(root->reloc_root == reloc_root);
 
 		/*
 		 * set reference count to 1, so btrfs_recover_relocation