btrfs-progs: restore: check if specified root is fs/file tree firstly

Steps to reproduce:
 # mkfs.btrfs -f /dev/sda9
 # mount /dev/sda9 /mnt
 # dd if=/dev/zero of=/mnt/data bs=1M count=1
 # btrfs restore -r /dev/sda9 -r 2 -o /tmp

If users don't input a valid fs/file root objectid, btrfs restore still
continue and don't restore anything, this is unfriendly, we could
check it firstly.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

cmds-restore.c

@@ -1186,6 +1186,11 @@ int cmd_restore(int argc, char **argv)
 				break;
 			case 'r':
 				root_objectid = arg_strtou64(optarg);
+				if (!is_fstree(root_objectid)) {
+					fprintf(stderr, "objectid %llu is not a valid fs/file tree\n",
+							root_objectid);
+					exit(1);
+				}
 				break;
 			case 'l':
 				list_roots = 1;