net: mvneta: alloc skb_shared_info on the mvneta_rx_swbm stack

Build skb_shared_info on mvneta_rx_swbm stack and sync it to xdp_buff
skb_shared_info area only on the last fragment. Leftover cache miss in
mvneta_swbm_rx_frame will be addressed introducing mb bit in
xdp_buff/xdp_frame struct

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>