ceph: add d_drop for some error cases in ceph_symlink()

When file num exceeds quota limit, should call d_drop to drop
dentry from cache as well.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>