net: tcp: Retransmit buffers are not freed on tcp_release

TCP maintains 'sent_list' for retransmission if it doesn't get ACK for it.
Same list is not freed on net_tcp_release() call. This causes memory leak.

Change-Id: I2b2def1ea19487cc48ea4fbb6343ef0c773f288f
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>