libbtrfsutil: don't check for UID 0 in subvolume_info()

btrfs_util_subvolume_info() explicitly checks whether geteuid() == 0 to
decide whether to use the unprivileged BTRFS_IOC_GET_SUBVOL_INFO ioctl
or the privileged BTRFS_IOC_TREE_SEARCH ioctl. This breaks in user
namespaces:

  $ unshare -r python3 -c 'import btrfsutil; print(btrfsutil.subvolume_info("/"))'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
  btrfsutil.BtrfsUtilError: [BtrfsUtilError 12 Errno 1] Could not search B-tree: Operation not permitted: '/'

The unprivileged ioctl has been supported since Linux 4.18. Let's try
the unprivileged ioctl first, then fall back to the privileged version
only if it isn't supported.

Signed-off-by: Omar Sandoval <osandov@fb.com>

Author: osandov

libbtrfsutil/subvolume.c

@@ -451,8 +451,10 @@ PUBLIC enum btrfs_util_error btrfs_util_subvolume_info_fd(int fd, uint64_t id,
 		if (err)
 			return err;
 
-		if (!is_root())
-			return get_subvolume_info_unprivileged(fd, subvol);
+		err = get_subvolume_info_unprivileged(fd, subvol);
+		if (err != BTRFS_UTIL_ERROR_GET_SUBVOL_INFO_FAILED ||
+		    errno != ENOTTY)
+			return err;
 
 		err = btrfs_util_subvolume_id_fd(fd, &id);
 		if (err)