regmap: regcache-rbtree: Clean new present bits on present bitmap resize

When inserting a new register into a block, the present bit map size is
increased using krealloc. krealloc does not clear the additionally
allocated memory, leaving it filled with random values. Result is that
some registers are considered cached even though this is not the case.

Fix the problem by clearing the additionally allocated memory. Also, if
the bitmap size does not increase, do not reallocate the bitmap at all
to reduce overhead.

Fixes: 3f4ff561

 ("regmap: rbtree: Make cache_present bitmap per node")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org