To optimize the layout of the meta data of mem_slab & mem_pool

[zephyrbot]

Reported by Quanwen Du:

The suggestions here:

1. For mem_slab, current logic is using the first 4bytes of the free buffer as NEXT pointer of free_list, then link all the free buffers together. But I suggest to use the tail 4bytes as NEXT pointer of free_list. As we know there are issues of memory overflow/corruption. If we move the NEXT to the tail 4bytes, then we have a bigger guard space, then even when memory overflow writing happens, it is lower possibility that the NEXT pointer will be corrupted. I know there is argument that we should let the corruption be visible earlier, but I think it shall be always right to reduce the impact of the overflow corruption
2. There is similar suggestion for mem_pool, which puts the bitmap and level info at the end of the buffer. So the suggestion here is to change K_MEM_POOL_DEFINE: to move _mpool_lvls_##name before _mpool_buf_##name; also to define the bitmap of block availability before mpool_buf##name. Of cause this suggestion assumed the toolchain will layout these arrays per its definition sequence, I will assume this is the most case, so shall be OK to re-layout these arrays.

(Imported from Jira ZEP-2435)