mon/PGMap: fix stored_raw calculation

mon/PGMap: fix stored_raw calculation

The get_user_bytes() helper is a bit weird because it uses the
raw_used_rate (replication/EC factor) so that it can work *backwards*
from raw usage to normalized user usage.  However, the legacy case that
works from PG stats does not use this factor... and the stored_raw value
(in the JSON output only) was incorrectly passing in a factor of 1.0,
which meant that for legacy mode it was a bogus value.

Fix by calculating stored_raw as stored_normalized * raw_used_rate.

Signed-off-by: Sage Weil <sage@redhat.com>