Set MALLOC_DEBUG_MAXZONES=1 in GENERIC-NODEBUG configurations.

The purpose of this option is to make it easier to track down memory
corruption bugs by reducing the number of malloc(9) types that might
have recently been associated with a given chunk of memory. However, it
increases fragmentation and is disabled in release kernels.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation