ping6: Ensure to initialize msghdr.

msg_flags was not initialized.  It is okay because it will not be
used by sendmsg(2), but it is a good practice to initialize
whole part of msghdr.

Bug noticed by Jan Synacek <jsynacek@redhat.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>