assoc_array: trim the final shortcut word using the current chunk end

assoc_array_walk() masks off the bits past shortcut->skip_to_level in the
word that contains skip_to_level, gated on
round_up(sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE) > skip_to_level.

That guard is wrong in two opposite ways:

 - When sc_level is word-aligned (every word after the first) round_up()
   is a no-op, so the guard is sc_level > skip_to_level and never fires for
   the word that holds skip_to_level.  A shortcut that spans more than one
   word and ends in the middle of its last word leaves that word untrimmed,
   and its stale high bits leak into the dissimilarity word and can steer
   the walk down the wrong descendant.

 - When sc_level is unaligned (the first word) and skip_to_level sits on
   the next chunk boundary, sc_level + CHUNK would exceed skip_to_level and
   fire the trim with shift = skip_to_level & CHUNK_MASK == 0, which clears
   the whole dissimilarity word and makes a differing shortcut compare
   equal.

Use the end of the chunk that contains sc_level instead:

	skip_to_level < round_down(sc_level, CHUNK) + CHUNK

For an aligned sc_level whose word holds skip_to_level this now fires (the
first bug); for an unaligned sc_level with skip_to_level on the following
boundary it does not, so shift is never 0 when the branch runs and the trim
never clears the whole word.

Fixes: 3cb989501c26 ("Add a generic associative array implementation.")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20260719161505.2423935-4-michael.bommarito@gmail.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

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