mm/damon/core: add damon_merge_two_regions() debug_sanity check

mm/damon/core: add damon_merge_two_regions() debug_sanity check

A data corruption could cause damon_merge_two_regions() creating zero
length DAMON regions.  Add a sanity check for that under
CONFIG_DAMON_DEBUG_SANITY.

Link: https://lkml.kernel.org/r/20260306152914.86303-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>