loader: Simplify build a little

[imp]

Summary

Confine -DDISK_DEBUG to biosdisc.c, the only file it affects.
Use modern variable arrays instead of alloca and add a sanity
size minimum for biospnp nodes. These nodes are tiny enough that
we needn't do a malloc/free pair: the stack is fine.

Sponsored by: Netflix

[tsoome]

alloca or local variable does allocate from stack (which may be in collision with bss), the opther option there to get low memory allocation is to use bio_alloc() and bio_free().

[imp]

In D43914#1001802, @tsoome wrote:

alloca or local variable does allocate from stack (which may be in collision with bss), the opther option there to get low memory allocation is to use bio_alloc() and bio_free().

Pnp nodes are like 32 bytes... and must be in the lower MB.

[tsoome]

In D43914#1001826, @imp wrote:

In D43914#1001802, @tsoome wrote:

alloca or local variable does allocate from stack (which may be in collision with bss), the opther option there to get low memory allocation is to use bio_alloc() and bio_free().

Pnp nodes are like 32 bytes... and must be in the lower MB.

Yep, same for bio_alloc - we currently do use it for disk io bounce buffer allocations.