simple_lmk: Fix broken multicopy atomicity for victims_to_kill

simple_lmk: Fix broken multicopy atomicity for victims_to_kill

When the reclaim thread writes to victims_to_kill on one CPU, it expects
the updated value to be immediately reflected on all CPUs in order for
simple_lmk_mm_freed() to work correctly. Due to the lack of memory
barriers to guarantee multicopy atomicity, simple_lmk_mm_freed() can be
given a victim's mm without knowing the correct victims_to_kill value,
which can cause the reclaim thread to remain stuck waiting forever for
all victims to be freed. This scenario, despite being rare, has been
observed.

Fix this by using proper atomic helpers with memory barriers.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: celtare21 <celtare21@gmail.com>