KFENCE: Clarify that sample allocations are not following NUMA or memory policies

KFENCE: Clarify that sample allocations are not following NUMA or memory policies

KFENCE manages its own pools and redirects regular memory allocations
to those pools in a sporadic way. The usual memory allocator features
like NUMA, memory policies and pfmemalloc are not supported.
This means that one gets surprising object placement with KFENCE that
may impact performance on some NUMA systems.

Update the description and make KFENCE depend on VM debugging
having been enabled.

Signed-off-by: Christoph Lameter <cl@linux.com>