net: mvneta: avoid copying shared_info frags in mvneta_swbm_build_skb

Avoid copying skb_shared_info frags array in mvneta_swbm_build_skb() since
__build_skb_around() does not overwrite it

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