mm/damon/core: avoid overflow in damon_feed_loop_next_input()

damon_feed_loop_next_input() is inefficient and fragile to overflows. 
Specifically, 'score_goal_diff_bp' calculation can overflow when 'score'
is high.  The calculation is actually unnecessary at all because 'goal' is
a constant of value 10,000.  Calculation of 'compensation' is again
fragile to overflow.  Final calculation of return value for under-achiving
case is again fragile to overflow when the current score is
under-achieving the target.

Add two corner cases handling at the beginning of the function to make the
body easier to read, and rewrite the body of the function to avoid
overflows and the unnecessary bp value calcuation.

Link: https://lkml.kernel.org/r/20241031161203.47751-1-sj@kernel.org


Fixes: 9294a037 ("mm/damon/core: implement goal-oriented feedback-driven quota auto-tuning")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/944f3d5b-9177-48e7-8ec9-7f1331a3fea3@roeck-us.net


Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: <stable@vger.kernel.org>	[6.8.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>