ceph: drop negative child dentries before try pruning inode's alias

Negative child dentry holds reference on inode's alias, it makes
d_prune_aliases() do nothing.

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