client: wake up umount waiter if receiving session open message

Wake up umount waiter if receiving session open message while
umounting. The umount waiter will re-close the session.

Fixes: #8004
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1633 from ceph/wip-8004

client: wake up umount waiter if receiving session open message

Reviewed-by: Sage Weil <sage@inktank.com>