`malloc_usable_size()` segfaults under the proxy library based on the jemalloc pool

[ldorau]

malloc_usable_size() segfaults under the proxy library based on the jemalloc pool.

When the proxy library is based on the scalable pool malloc_usable_size() works correctly.

Ref: #883

Root cause:

op_malloc_usable_size() in pool_jemalloc calls malloc_usable_size@libumf_proxy instead of malloc_usable_size@jemalloc what causes an infinite recursive loop of function calls and a segfault as a result.