mon/PGMap: don't mangle stamp_delta in clear_delta()

This is a delta, not a timestamp.

This triggered when a cluster is idle for 2* the mon_delta_reset_interval,
and required a mon restart to fix.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>