regmap: maple: Fix cache corruption in regcache_maple_drop()

regmap: maple: Fix cache corruption in regcache_maple_drop()

When keeping the upper end of a cache block entry, the entry[] array
must be indexed by the offset from the base register of the block,
i.e. max - mas.index.

The code was indexing entry[] by only the register address, leading
to an out-of-bounds access that copied some part of the kernel
memory over the cache contents.

This bug was not detected by the regmap KUnit test because it only
tests with a block of registers starting at 0, so mas.index == 0.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: f033c26de5a5 ("regmap: Add maple tree based register cache")
Link: https://msgid.link/r/20240327114406.976986-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>