lib: packing: add pack() and unpack() wrappers over packing()

Geert Uytterhoeven described packing() as "really bad API" because of
not being able to enforce const correctness. The same function is used
both when "pbuf" is input and "uval" is output, as in the other way
around.

Create 2 wrapper functions where const correctness can be ensured.
Do ugly type casts inside, to be able to reuse packing() as currently
implemented - which will _not_ modify the input argument.

Also, take the opportunity to change the type of startbit and endbit to
size_t - an unsigned type - in these new function prototypes. When int,
an extra check for negative values is necessary. Hopefully, when
packing() goes away completely, that check can be dropped.

My concern is that code which does rely on the conditional directionality
of packing() is harder to refactor without blowing up in size. So it may
take a while to completely eliminate packing(). But let's make alternatives
available for those who do not need that.

Link: https://lore.kernel.org/netdev/[email protected]/
Signed-off-by: Vladimir Oltean <[email protected]>
Signed-off-by: Jacob Keller <[email protected]>
Reviewed-by: Przemek Kitszel <[email protected]>
Reviewed-by: Vladimir Oltean <[email protected]>
Link: https://patch.msgid.link/20241002-packing-kunit-tests-and-split-pack-unpack-v2-4-8373e551eae3@intel.com
Signed-off-by: Jakub Kicinski <[email protected]>

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