gro: Fix use after free in tcp_gro_receive
After calling skb_gro_receive skb->len can no longer be relied
on since if the skb was merged using frags, then its pages will
have been removed and the length reduced.
This caused tcp_gro_receive to prematurely end merging which
resulted in suboptimal performance with ixgbe.
The fix is to store skb->len on the stack.
Reported-by: Mark Wagner <mwagner@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>