nfs: remove timespec from xdr_encode_nfstime

For NFSv2 and NFSv3, timestamps are stored using 32-bit entities
and overflow in y2038. For historic reasons we truncate the
64-bit timestamps by converting from a timespec64 to a timespec
first.

Remove this unnecessary conversion step and do the truncation
in the final functions that take a timestamp.

This is transparent to users, but avoids one of the last uses
of 'timespec' and lets us remove it later.

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