lib: packing: use BITS_PER_BYTE instead of 8

This helps clarify what the 8 is for.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241002-packing-kunit-tests-and-split-pack-unpack-v2-9-8373e551eae3@intel.com


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