kasan: monitor pools

[Maxime Villard <max%m00nbsd.net@localhost>]

Here is a patch [1] that allows kasan to monitor pools and pool_caches. We
recycle the existing POOL_REDZONE implementation - which I wrote three years
ago, and which has never been enabled (not even on DEBUG). With this we can
detect read/write buffer overflows on all our pools, and in particular, on
mbufs.

I guess people are fine? Otherwise if we prefer to keep a KASAN-independent
version of POOL_REDZONE, then we need to enable it under DIAGNOSTIC at least
for it to be meaningful.

[1] http://m00nbsd.net/garbage/kasan/pool.diff