lib: packing: use GENMASK() for box_mask

This is an u8, so using GENMASK_ULL() for unsigned long long is
unnecessary.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20241002-packing-kunit-tests-and-split-pack-unpack-v2-10-8373e551eae3@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>