ceph: do_sync is never initialized

Probably this code was syncing a lot more often then intended because
the do_sync variable wasn't set to zero.

Cc: stable@vger.kernel.org # v3.11+
Fixes: c62988ec ('ceph: avoid meaningless calling ceph_caps_revoking if sync_mode == WB_SYNC_ALL.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@redhat.com>