prctl: prctl_set_mm -- Bring back handling of PR_SET_MM_x

Hi! Yesterday I've been pointed out that we unfortunatelly can't drop off PR_SET_MM_x operations. So here is a series which bring them back but instead of plain revert I tried to simplify the code and make it as minimum as possible.

WARN: the series is compile-tested only for a while, I wanted to share it early so maybe someone has better idea of how integrate old PR_SET_MM_x operations. I'll do full tests today hopefully.

Cyrill