netfilter: xt_qtaguid: fix ipv6 protocol lookup

When updating the stats for a given uid it would incorrectly assume
IPV4 and pick up the wrong protocol when IPV6.

Change-Id: Iea4a635012b4123bf7aa93809011b7b2040bb3d5
Signed-off-by: JP Abgrall <jpa@google.com>