net: ipv6: add missing lock in ping_v6_sendmsg

[net-next commit a1bdc45580fc19e968b32ad27cd7e476a4aa58f6]

Bug: 9469865
Change-Id: I7ae28d29c645c535e570eb8d12f45e8eafd9c70b
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