mempool: implement malloc_init

mempool: implement malloc_init

Add a *_malloc_init to the API to allow allocating memory initialized to
a given initial value.

This replaces application patterns where a loop on all possible CPUs is
needed after allocation to initialize memory to a given nonzero value,
and will eventually allow doing the initialization lazily (the first
time the cpu touches memory within the pool).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I3acb1505d7b4f46ceb6a8128d2ebda346b8e359c