dm-stats: fix merge accounting

There were wrong parentheses when setting stats_aux->merged, so that
merging was never properly accounted. This commit fixes it.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Fixes: fd2ed4d25270 ("dm: add statistics support")
Cc: stable@vger.kernel.org