net: ping: make ping_v6_sendmsg static

As ping_v6_sendmsg is used only in this file,
making it static

The body of "pingv6_prot" and "pingv6_protosw" were
moved at the middle of the file, to avoid having to
declare some static prototypes.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>