mempool: simplify the logic for sizing the free block bitmap

Z_MPOOL_LVLS() expands into the sum of 16 _MPOOL_HAVE_LVL() instances,
and _MPOOL_BITS_SIZE() expands into the sum of 16 Z_MPOOL_LVLS()
instances. In the end, a single _MPOOL_BITS_SIZE() expands to 256
_MPOOL_HAVE_LVL() instances!

Let's make it slightly easier on the compiler, and easier for humans
too, by reworking Z_MPOOL_HAVE_LVL(() so that ic can be used directly
into Z_MPOOL_LBIT_BYTES(), making the code logic much simpler.

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