cifs: use timespec64 internally

In cifs, the timestamps are stored in memory in the cifs_fattr structure,
which uses the deprecated 'timespec' structure. Now that the VFS code
has moved on to 'timespec64', the next step is to change over the fattr
as well.

This also makes 32-bit and 64-bit systems behave the same way, and
no longer overflow the 32-bit time_t in year 2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>