net: ipv6: add missing lock in ping_v6_sendmsg

[net-next commit a1bdc45580fc19e968b32ad27cd7e476a4aa58f6]

Bug: 9469865
Change-Id: I7ae28d29c645c535e570eb8d12f45e8eafd9c70b
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Conflicts:
	net/ipv6/ping.c