lib: packing: adjust definitions and implementation for arbitrary buffer lengths

Jacob Keller has a use case for packing() in the intel/ice networking
driver, but it cannot be used as-is.

Simply put, the API quirks for LSW32_IS_FIRST and LITTLE_ENDIAN are
naively implemented with the undocumented assumption that the buffer
length must be a multiple of 4. All calculations of group offsets and
offsets of bytes within groups assume that this is the case. But in the
ice case, this does not hold true. For example, packing into a buffer
of 22 bytes would yield wrong results, but pretending it was a 24 byte
buffer would work.

Rather than requiring such hacks, and leaving a big question mark when
it comes to discontinuities in the accessible bit fields of such buffer,
we should extend the packing API to support this use case.

It turns out that we can keep the design in terms of groups of 4 bytes,
but also make it work if the total length is not a multiple of 4.
Just like before, imagine the buffer as a big number, and its most
significant bytes (the ones that would make up to a multiple of 4) are
missing. Thus, with a big endian (no quirks) interpretation of the
buffer, those most significant bytes would be absent from the beginning
of the buffer, and with a LSW32_IS_FIRST interpretation, they would be
absent from the end of the buffer. The LITTLE_ENDIAN quirk, in the
packing() API world, only affects byte ordering within groups of 4.
Thus, it does not change which bytes are missing. Only the significance
of the remaining bytes within the (smaller) group.

No change intended for buffer sizes which are multiples of 4. Tested
with the sja1105 driver and with downstream unit tests.

Link: https://lore.kernel.org/netdev/[email protected]/
Signed-off-by: Vladimir Oltean <[email protected]>
Tested-by: Jacob Keller <[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-v2-2-8373e551eae3@intel.com
Signed-off-by: Jakub Kicinski <[email protected]>

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