skbuff: rename sock_zerocopy_* to msg_zerocopy_*

At Willem's suggestion, rename the sock_zerocopy_* functions
so that they match the MSG_ZEROCOPY flag, which makes it clear
they are specific to this zerocopy implementation.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>