vmem: add vmem_malloc_usable_size() to API

Internally, *je_malloc_usable_size* has different path for small allocations, and for huge allocations. (It detects huge allocations by checking if *ptr* is aligned to chunk size (4MB)).
To determine the usable size of huge allocation, jemalloc must traverse the allocation tree (for each pool) to find given *ptr.* So, it looks like passing *vmp* down to *je_malloc_usable_size*, would allow jemalloc to scan only the allocation tree associated with given pool.