malloc: make heap a doubly-linked list

malloc: make heap a doubly-linked list

As we are preparing for dynamic memory allocation, we need to be
able to handle holes in our malloc heap, hence we're switching to
doubly linked list, and prepare infrastructure to support it.

Since our heap is now aware where are our first and last elements,
there is no longer any need to have a dummy element at the end of
each heap, so get rid of that as well. Instead, let insert/remove/
join/split operations handle end-of-list conditions automatically.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>