linux/mips.zig: Use i32 for stat nsec fields

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`.

Author: rootbeer

lib/std/os/linux/mips.zig

@@ -317,14 +317,14 @@ pub const Stat = extern struct {
     uid: uid_t,
     gid: gid_t,
     rdev: dev_t,
-    __pad1: [2]u32, // -1 because our dev_t is u64 (kernel dev_t is really u32).
+    __pad1: [2]u32,
     size: off_t,
     atim: i32,
-    atim_nsec: u32,
+    atim_nsec: i32,
     mtim: i32,
-    mtim_nsec: u32,
+    mtim_nsec: i32,
     ctim: i32,
-    ctim_nsec: u32,
+    ctim_nsec: i32,
     blksize: blksize_t,
     __pad3: u32,
     blocks: blkcnt_t,