The functions that are returning tcp sequence number also setup
TS offset value, so rename them to better describe their purpose.

No functional changes in this patch.
Suggested-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NAlexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>