nfs: use timespec64 in nfs_fattr

Push down the use of timespec64 into NFS nfs_fattr, to avoid needless
conversions, and get closer to having 64-bit time_t support on 32-bit
NFSv4 and removing some old interfaces from the kernel.

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