ext4: dont return EINVAL from GETFSUUID when reporting UUID length

If userspace calls this ioctl with fsu_length (the length of the
fsuuid.fsu_uuid array) set to zero, ext4 copies the desired uuid length
out to userspace.  The kernel call returned a result from a valid input,
so the return value here should be zero, not EINVAL.

While we're at it, fix the copy_to_user call to make it clear that we're
only copying out fsu_len.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Catherine Hoang <catherine.hoang@oracle.com>
Link: https://lore.kernel.org/r/166811138914.327006.9241306894437166566.stgit@magnoliaSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ioctl.c

@@ -1154,9 +1154,10 @@ static int ext4_ioctl_getuuid(struct ext4_sb_info *sbi,
 
 	if (fsuuid.fsu_len == 0) {
 		fsuuid.fsu_len = UUID_SIZE;
-		if (copy_to_user(ufsuuid, &fsuuid, sizeof(fsuuid.fsu_len)))
+		if (copy_to_user(&ufsuuid->fsu_len, &fsuuid.fsu_len,
+					sizeof(fsuuid.fsu_len)))
 			return -EFAULT;
-		return -EINVAL;
+		return 0;
 	}
 
 	if (fsuuid.fsu_len != UUID_SIZE || fsuuid.fsu_flags != 0)