ceph: fix dentry leak in splice_dentry()

In any case, d_splice_alias() does not drop reference of original
dentry.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>