apparmor: fix fmt string type error in process_strs_entry

apparmor: fix fmt string type error in process_strs_entry

pointer subtraction has a type of int when using clang on hexagon,
microblaze (and possibly other archs). We know the subtraction is
postive so cast the expression to unsigned long to match what is in
the fmt string.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602021429.CcmWkR9K-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202602021427.PvvDjgyL-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202602021510.JPzX5zKb-lkp@intel.com/
Fixes: c140dcd1246bf ("apparmor: make str table more generic and be able to have multiple entries")
Signed-off-by: John Johansen <john.johansen@canonical.com>