malloc: separate creating memseg list and malloc heap

malloc: separate creating memseg list and malloc heap

Currently, creating external malloc heap involves also creating
a memseg list backing that malloc heap. We need to have them as
separate functions, to allow creating memseg lists without
creating a malloc heap.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>