nfs: encode nfsv4 timestamps as 64-bit

On 32-bit architectures, xdr_encode_nfstime4() needlessly
truncates timestamps to a 32-bit value in the range between
year 1902 and 2038.

Change it to use 'struct timespec64' to allow the entire range
of values supported by the server.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>