The code is doing a split in split_alloc(), adding the leftover to the
free list, then splitting the suffix away in sys_heap_aligned_alloc(),
removing the former leftover from the free list, combining it with the
suffix and finally adding the combined chunk back to the free list.
Instead, let's have each allocator do their own splitting only once by
moving the split_alloc() processing upstream rather than downstream.
This also allows for the "used" flag to be set only once at the end
rather than being overwritten along the way.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>