ceph: fix dentry leak in ceph_readdir_prepopulate

[ Upstream commit c58f450b ]

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I3e0f7351ebb94976c2a1952ab151a5b019602b52