The `atime()`, etc wrappers here expect to create a `std.linux.timespec`
(defined in `linux.zig` to have `isize` fields), so the u32 causes errors:

   error: expected type 'isize', found 'u32'
   .nsec = self.atim_nsec,

Make the nsec fields signed for consistency with all the other structs,
with and with `std.linux.timespec`.

Also looks like the comment on `__pad1` was copied from `__pad0`, but it
only applies to `__pad0`.