fcntl14: dup code clean up

And block3 description message fix from "mandatory locking"
to "negative whence".

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
fcntl14: only skip test block2 on NFS

NFS does not support mandatory locking.

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>