net: mvneta: move skb build after descriptors processing

Move skb build after all descriptors processing. This is a preliminary
patch to enable multi-buffers and JUMBO frames support for XDP.
Introduce mvneta_xdp_put_buff routine to release all pages used by a
XDP multi-buffer

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>