net: tcp2: Refactor tcp_out()

In order to support the data retransmission, refactor tcp_out()
into tcp_out_ext() which supports passing the sequence number.

In addition drop modifications of the connection sequence number
from tcp_out_ext(), this is the responsibility of data retransmission.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>