[TCP]: Fix TCP_OFF() bug check introduced by previous change.

The TCP_OFF assignment at the bottom of that if block can indeed set
TCP_OFF without setting TCP_PAGE.  Since there is not much to be
gained from avoiding this situation, we might as well just zap the
offset.  The following patch should fix it.

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