skbuff: Add skb parameter to the ubuf zerocopy callback

Add an optional skb parameter to the zerocopy callback parameter,
which is passed down from skb_zcopy_clear().  This gives access
to the original skb, which is needed for upcoming RX zero-copy
error handling.

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