tcp: pass previous skb to tcp_shifted_skb()

No need to recompute previous skb, as it will be a bit more
expensive when rtx queue is converted to RB tree.

Change-Id: Ia23efc666ff335c2086759896d6e4491f9b3f1aa
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: f33198163a0fbb03766444253edf6ea50685d725
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Suresh H A <hasuresh@codeaurora.org>