net: ipv6: fix wrong ping_v6_sendmsg return value

[msm/android-3.4 commit 5f0fd6105f491b8b435863e98f7e96d617e3da84]

ping_v6_sendmsg currently returns 0 on success. It should return
the number of bytes written instead.

Bug: 9469865
Change-Id: Ic5f3eeb74725bfb5bdca9a533461492b68e2e072
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>