AX.25: Close socket connection on session completion

AX.25: Close socket connection on session completion

[ Upstream commit 4a7d99ea1b27734558feb6833f180cd38a159940 ]

A socket connection made in ax.25 is not closed when session is
completed.  The heartbeat timer is stopped prematurely and this is
where the socket gets closed. Allow heatbeat timer to run to close
socket. Symptom occurs in kernels >= 4.2.0

Originally sent 6/15/2016. Resend with distribution list matching
scripts/maintainer.pl output.

Signed-off-by: Basil Gunn <basil@pacabunga.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>