Chase malloc() change by removing lg_chunk malloc_conf settings.
In jemalloc 5, there are no longer chunks, and as configured on
FreeBSD (the "retain" option defaults to false), the mmap()
requests are precisely sized for the specific needs, which means
the virtual memory overhead should be lower for small applications.
Reviewed by: jasone, ian
Differential Revision: https://reviews.freebsd.org/D11366