Preserve CHUNK_MAP_UNZEROED for small runs.

Preserve CHUNK_MAP_UNZEROED when allocating small runs, because it is
possible that untouched pages will be returned to the tree of clean
runs, where the CHUNK_MAP_UNZEROED flag matters.  Prior to the
conversion from CHUNK_MAP_ZEROED, this was already a bug, but in the
worst case extra zeroing occurred.  After the conversion, this bug made
it possible to incorrectly treat pages as pre-zeroed.