malloc: move malloc_type_internal into malloc_type

According to code comments the original motivation was to allow for
malloc_type_internal changes without ABI breakage. This can be trivially
accomplished by providing spare fields and versioning the struct, as
implemented in the patch below.

The upshots are one less memory indirection on each alloc and disappearance
of mt_zone.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D27104