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: 12800827
Change-Id: I7ed17dc61afbb68a84908e67e44db976ec812bad
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>