ceph: flush pending works before shutdown super

ceph: flush pending works before shutdown super

BugLink: https://bugs.launchpad.net/bugs/1834235
Pending works hold inode references, which cause "Busy inodes after
unmount" warning.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit a57d9064e4ee4e9882b922d0627be3d426004c69)
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>