nfsd: Decode and send 64bit time values

The seconds field of an nfstime4 structure is 64bit, but we are assuming
that the first 32bits are zero-filled.  So if the client tries to set
atime to a value before the epoch (touch -t 196001010101), then the
server will save the wrong value on disk.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>