[IPV4/IPV6]: More fragment handling improvements.

[IPV4/IPV6]: More fragment handling improvements.

Here's the optimization: if we know the remaining data exceeds the
mtu, we do not need to fill up full of it.

skb_prev:
                mtu
+----------+--+-+
|          |  | |
+----------+--+-+
           ^  ^
skb_prev->len |
              maxfraglen

appending data:
           +--------+
           |        |
           +--------+
           --------->
                  length

In this case, we know we need more fragment(s).
So, let's fill up to maxfraglen (instead of mtu)
to avoid needless copy in the next loop.

Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>