net/act_pedit: fix an error code

I'm reviewing static checker warnings where we do ERR_PTR(0), which is
the same as NULL.  I'm pretty sure we intended to return ERR_PTR(-EINVAL)
here.  Sometimes these bugs lead to a NULL dereference but I don't
immediately see that problem here.

Fixes: 71d0ed70 ("net/act_pedit: Support using offset relative to the conventional network headers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>