NFS: Move internal constants out of uapi/linux/nfs_mount.h

When the label says "for internal use only", then it doesn't belong
in the 'uapi' subtree.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>