use proper 64bit casting for pointer-math

use proper 64bit casting for pointer-math

building with "./configure --enable-checking --enable-supergroups"
fails, because on 64bit, the casting in map.c gives a warning.
Just add another ifdef and cast according to the size of a pointer.

Reviewed-on: http://gerrit.openafs.org/3341
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit acdef43d818ee56e6c1f1dc29f508c02793c8355)
Change-Id: If7dbe6f41f015025447e68eb1fc9ea14e56d6fe2
Reviewed-on: http://gerrit.openafs.org/3496
Tested-by: Derrick Brashear <shadow@dementia.org>