Reduce LinkerSmallObjectAllocator memory overhead

The current implementation of LinkerSmallObjectAllocator keeps record
of pages in a vector, which uses its own page(s).  This is at least a
page overhead per LinkerSmallObjectAllocator.

This change removes the page record vector by managing the pages in a
doubly linked list.

We also fix a bug where we are actually keeping up to 2 free pages
instead of just one.

The memory used by small objects when running 'dd', before this change:
    72 KB  [anon:linker_alloc_small_objects]
    28 KB  [anon:linker_alloc_vector]

After this change:
    60 KB  [anon:linker_alloc_small_objects]

Test: Boot cuttlefish and check memory used by linker.
Change-Id: I3468fa4d853c78b4bc02bfb84a3531653f74fb17