netfilter: ipset: Fix an error code in ip_set_sockfn_get()

The copy_to_user() function returns the number of bytes remaining to be
copied.  In this code, that positive return is checked at the end of the
function and we return zero/success.  What we should do instead is
return -EFAULT.

Fixes: a7b4f989 ("netfilter: ipset: IP set core support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>