lib/os/heap: rename struct z_heap.len to struct z_heap.end_chunk

The end marker chunk was represented by the len field of struct z_heap.
It is now renamed to end_chunk to make it more obvious what it is.

And while at it...

Given that it is used in size_too_big() to cap the allocation size
already, we no longer need to test the bucket index against the
biggest index possible derived from end_chunk in alloc_chunk(). The
corresponding bucket_idx() call is relatively expensive on some
architectures so avoiding it (turning it into a CHECK() instead) is
a good thing.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>