btrfs: hold a ref on the root in merge_reloc_roots

We look up the corresponding root for the reloc root, we need to hold a
ref while we're messing with it.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2592,9 +2592,11 @@ void merge_reloc_roots(struct reloc_control *rc)
 			root = read_fs_root(fs_info,
 					    reloc_root->root_key.offset);
 			BUG_ON(IS_ERR(root));
+			BUG_ON(!btrfs_grab_fs_root(root));
 			BUG_ON(root->reloc_root != reloc_root);
 
 			ret = merge_reloc_root(rc, root);
+			btrfs_put_fs_root(root);
 			if (ret) {
 				if (list_empty(&reloc_root->root_list))
 					list_add_tail(&reloc_root->root_list,