Fix a regression in CHUNK_MAP_UNZEROED change.

Fix a regression added by revision:

	3377ffa1
	Change CHUNK_MAP_ZEROED to CHUNK_MAP_UNZEROED.

A modified chunk->map dereference was missing the subtraction of
map_bias, which caused incorrect chunk map initialization, as well as
potential corruption of the first non-header page of memory within each
chunk.