net: allow better page reuse in splice(sock -> pipe)

splice() from socket to pipe needs linear_to_page() helper to transfert
skb header to part of page.

We can reset the offset in the current sk->sk_sndmsg_page if we are the
last user of the page.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>