ceph: flush dirty caps of unlinked inode ASAP

Client should release unlinked inode from its cache ASAP. But client
can't release inode with dirty caps.

Link: http://tracker.ceph.com/issues/22886


Signed-off-by: Zhi Zhang <zhang.david2011@gmail.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>