ping: support ipv6 ping socket flow labels

Ping sockets don't appear to make any attempt to preserve flow labels
created and set by userspace using IPV6_FLOWINFO_SEND. Instead they are
clobbered by autolabels (if enabled) or zero.

Grab the flowlabel out of the msghdr similar to how rawv6_sendmsg does
it and move the memset up so it doesn't get zeroed after.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>