NFSv4 only print the label when its queried

When the bitmask of the attributes doesn't include the security label,
don't bother printing it. Since the label might not be null terminated,
adjust the printing format accordingly.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>