lib: packing: duplicate pack() and unpack() implementations

packing() is now used in some hot paths, and it would be good to get rid
of some ifs and buts that depend on "op", to speed things up a little bit.

With the main implementations now taking size_t endbit, we no longer
have to check for negative values. Update the local integer variables to
also be size_t to match.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241002-packing-kunit-tests-and-split-pack-unpack-v2-5-8373e551eae3@intel.com


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