skbuff: Rename skb_zcopy_{get|put} to net_zcopy_{get|put}

Unlike the rest of the skb_zcopy_ functions, these routines
operate on a 'struct ubuf', not a skb.  Remove the 'skb_'
prefix from the naming to make things clearer.

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