lib: sbi_heap: Allocate list nodes dynamically

Currently the heap has a fixed housekeeping factor of 16, which means
1/16 of the heap is reserved for list nodes. But this is not enough when
there are many small allocations; in the worst case, 1/3 of the heap is
needed for list nodes (32 byte heap_node for each 64 byte allocation).
This has caused allocation failures on some platforms.

Let's avoid trying to guess the best ratio. Instead, allocate more nodes
as needed. To avoid recursion, the nodes are permanent allocations. So
to minimize fragmentation, allocate them in small batches from the end
of the last free space node. Bootstrap the free space list by embedding
one node in the heap control struct.

Some error paths are avoided because the nodes are allocated up front.

Signed-off-by: Samuel Holland <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Tested-by: Anup Patel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Anup Patel <[email protected]>

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