lib/os/mempool: Fix corruption case with block splitting

The block_fits() predicate was borked.  It would check that a block
fits within the bounds of the whole heap.  But that's not enough:
because of alignment changes between levels the sub-blocks may be
adjusted forward.  It needs to fit inside the PARENT block that it was
split from.

What could happen at runtime is that the last subblocks of a
misaligned parent block would overlap memory from subsequent blocks,
or even run off the end of the heap.  That's bad.

Change the API of block_fits() a little so it can extract the parent
region and do this properly.

Fixes #15279.  Passes test introduced in #16728 to demonstrate what
seems like the same issue.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>