net: ena: Put orthogonal fields in ena_tx_buffer in a union

net: ena: Put orthogonal fields in ena_tx_buffer in a union

The skb and xdpf pointers cannot be set together in the driver
(each TX descriptor can send either an SKB or an XDP frame), and so it
makes more sense to put them both in a union.

This decreases the overall size of the ena_tx_buffer struct which
improves cache locality.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
Link: https://lore.kernel.org/r/20240101190855.18739-4-darinzon@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>