kernel: heap: Add allocation metadata to allocated_bytes

The allocated_bytes were missing the allocation metadata. Because of that
the sum of free_bytes and allocated_bytes doesn't remain constant after
each allocation. This convention doesn't match glibc's behavior. This
commit changes the chunksz_to_bytes function to include the metadata in the
calculation. The analysis of the mallinfo2 function from glibc has been
done in #92392 Pull Request.

Signed-off-by: Krzysztof Sychla <ksychla@antmicro.com>