IB/mlx4: Fix bug unwinding on error in mlx4_ib_init_sriov()

We have to decrement "i" before calling mlx4_ib_free_demux_ctx() or we
free something that wasn't allocated.  That's fine for free_pv_object()
but it would lead to a NULL dereference calling mlx4_ib_free_demux_ctx().
The null dereference is because ->tun is NULL when we check:

	if (!ctx->tun[i])

Also we didn't free ->sriov.demux[0] so it was a small leak.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>