[TCP]: Unite identical code from two seqno split blocks

Bogus seqno compares just mislead, the code is identical for
both sides of the seqno compare (and was even executed just
once because of return in between).

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>