malloc: do not count a MemGuard allocation in the type stats

[nick_spun.io]

Summary

uma_zalloc_arg() and uma_zalloc_domain() can return a MemGuard
allocation for a sampled zone. malloc() counted it with
malloc_type_zone_allocated(), but free() routes any MemGuard address to
memguard_free() through free_dbg() and never reaches
malloc_type_freed(), so the type's allocated total grew without a
matching free.

MemGuard allocations selected by malloc type already skip the accounting
on both sides. Skip it for a MemGuard address too, so the zone-selected
ones balance as well.

[nick_spun.io]

only added this as a child of D60608 because that's where I imported the headers and this will not land cleanly on its own