nfsd: use timespec64 in encode_time_delta

The values in encode_time_delta are always small and don't
overflow the range of 'struct timespec', so changing it has
no effect.

Change it to timespec64 as a prerequisite for removing the
timespec definition later.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>