fs: use timespec64 in relatime_need_update

For some reason, the conversion of the VFS code away from 'struct timespec'
left one function behind that still uses it, for absolutely no reason.

Using timespec64 will make the atime update logic work correctly past
y2038.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>