rgw: fix potential realm watch lost

rgw: fix potential realm watch lost

realm watch only restart on -ENOTCONN, but if we fail to maintain the watch ping, the OSD will return -ETIMEDOUT.
The objecter will keep the watch err on last_error, if last_error not empty, it will not call handle_error in future error,
so it will never get the chance to rewatch on following -ENOTCONN error.

Fixes: http://tracker.ceph.com/issues/40991
Signed-off-by: Tianshan Qu <tianshan@xsky.com>
(cherry picked from commit c2d6cbe1b1f4b7c26fa8da4eefa291b4bdb48835)