osdc/Objecter: handle race between calc_target and handle_osd_map

osdc/Objecter: handle race between calc_target and handle_osd_map

If we fail to get an existing session and have to take the exclusive lock,
we may race with an OSDMap update and end up with a stale target.  Check
for an epoch change and, if it happens, recalculate the mapping.

Fixes: http://tracker.ceph.com/issues/17942
Reported-by: wangdongxu <wangdongxu@cmss.chinamobile.com>
Reported-by: menglingkun <menglingkun@cmss.chinamobile.com>
Signed-off-by: Sage Weil <sage@redhat.com>