sbin/hammer: Optimize freemap initialization

sbin/hammer: Optimize freemap initialization

Bring in the way hammer_iterate_l1l2_entries() initializes
layer1/2 to initialize_freemap(). The layer1 crc needs to
be updated only once in every 2^19 layer2 entries. No need
to do it for every layer2 entry as they'll be overwritten
except for the one after the last layer2 (2^19th layer2 entry)
within layer1.

Also having nested loops here for layer1 and layer2 seems
to better explain the concept of hammer's blockmap.