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/zfs/zfs.c

@@ -3771,7 +3771,7 @@ zfs_uuid (grub_device_t device, char **uuid)
 }
 
 static grub_err_t 
-zfs_mtime (grub_device_t device, grub_int32_t *mt)
+zfs_mtime (grub_device_t device, grub_int64_t *mt)
 {
   struct grub_zfs_data *data;
   grub_zfs_endian_t ub_endian = GRUB_ZFS_UNKNOWN_ENDIAN;