net: tcp: Cancel the fin_timer on FIN message in FIN_WAIT1 state

fin_timer will be started after sending FIN to peer. After successful
reception of FIN+ACK message in FIN_WAIT1 state, fin_timer should be
cancelled.

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