net: mvneta: fix possible use-after-free in mvneta_xdp_put_buff

Release first buffer as last one since it contains references
to subsequent fragments. This code will be optimized introducing
multi-buffer bit in xdp_buff structure.

Fixes: ca0e0146 ("net: mvneta: move skb build after descriptors processing")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>