osd: initialize send_notify on pg load

osd: initialize send_notify on pg load

When the PG is loaded, we need to set send_notify if we are not the
primary.  Otherwise, if the PG does not go through
start_peering_interval() or experience a role change, we will not set
the flag and tell the primary that we exist.  This can cause problems
for example if we have unfound objects that the primary needs, although
I'm sure there are other bad implications as well.

Fixes: #2866
Signed-off-by: Sage Weil <sage@inktank.com>