net: fix napi_reuse_skb() skb reserve

napi->skb is allocated in napi_get_frags() using
netdev_alloc_skb_ip_align(), with a reserve of NET_SKB_PAD +
NET_IP_ALIGN bytes.

However, when such skb is recycled in napi_reuse_skb(), it ends with a
reserve of NET_IP_ALIGN which is suboptimal.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>