ext4: dont return EINVAL from GETFSUUID when reporting UUID length

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@magnolia
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org