fs: Use 64-bit type for filesystem timestamp

Some filesystems nowadays use 64-bit types for timestamps. So, update
grub_dirhook_info struct to use an grub_int64_t type to store mtime.
This also updates the grub_unixtime2datetime() function to receive
a 64-bit timestamp argument and do 64-bit-safe divisions.

All the remaining conversion from 32-bit to 64-bit should be safe, as
32-bit to 64-bit attributions will be implicitly casted. The most
critical part in the 32-bit to 64-bit conversion is in the function
grub_unixtime2datetime() where it needs to deal with the 64-bit type.
So, for that, the grub_divmod64() helper has been used.

These changes enables the GRUB to support dates beyond y2038.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/squash4.c

@@ -1003,7 +1003,7 @@ grub_squash_close (grub_file_t file)
 }
 
 static grub_err_t
-grub_squash_mtime (grub_device_t dev, grub_int32_t *tm)
+grub_squash_mtime (grub_device_t dev, grub_int64_t *tm)
 {
   struct grub_squash_data *data = 0;