sched/fair: Cleanup in migrate_degrades_locality() to improve readability

migrate_degrade_locality() would return {1, 0, -1} respectively to
indicate that migration would degrade-locality, would improve
locality, would be ambivalent to locality improvements.

This patch improves readability by changing the return value to mean:
* Any positive value degrades locality
* 0 migration doesn't affect locality
* Any negative value improves locality

[Swapnil: Fixed comments around code and wrote commit log]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Not-yet-signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241220063224.17767-3-swapnil.sapkal@amd.com