net: mvneta: simplify mvneta_swbm_add_rx_fragment management

Relying on xdp frags bit, remove skb_shared_info structure
allocated on the stack in mvneta_rx_swbm routine and simplify
mvneta_swbm_add_rx_fragment accessing skb_shared_info in the
xdp_buff structure directly. There is no performance penalty in
this approach since mvneta_swbm_add_rx_fragment is run just
for xdp frags use-case.

Acked-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/45f050c094ccffce49d6bc5112939ed35250ba90.1642758637.git.lorenzo@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>