malloc: fix heap size not set on init

malloc: fix heap size not set on init

When heap initializes, we need to add already allocated segments
onto the heap. However, in doing that, we never increased total
heap size. Fix it by adding segment length to total heap length
when initializing the heap.

Fixes: 66cc45e293ed ("mem: replace memseg with memseg lists")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>