net: ipv6: fix wrong ping_v6_sendmsg return value

[net-next commit fbfe80c890a1dc521d0b629b870e32fcffff0da5]

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

Bug: 9469865
Change-Id: I86eb936e06bf8582975d59597e48e2bcc53b958d
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>