osd: reset session->osdmap if session is not waiting for a map anymore

osd: reset session->osdmap if session is not waiting for a map anymore

we should release the osdmap reference once we are done with it,
otherwise we might need to wait very long to update that reference with
a newer osdmap ref. this appears to be an OSDMap leak: it is held by an
quiet OSD::Session forever.

the osdmap is not reset in OSD::session_notify_pg_create(), because its
only caller is wake_pg_waiters(), which will call
dispatch_session_waiting() later. and dispatch_session_waiting() will
check the session->osdmap, and will also reset the osdmap if
session->waiting_for_pg.empty().

Fixes: http://tracker.ceph.com/issues/13990
Signed-off-by: Kefu Chai <kchai@redhat.com>