[malloc] Ensure free memory blocks remain aligned

[malloc] Ensure free memory blocks remain aligned

When allocating memory with a non-zero alignment offset, the free
memory block structure following the allocation may end up improperly
aligned.

Ensure that free memory blocks always remain aligned to the size of
the free memory block structure.

Ensure that the initial heap is also correctly aligned, thereby
allowing the logic for leaking undersized free memory blocks to be
omitted.

Signed-off-by: Michael Brown <mcb30@ipxe.org>