Description of problem:

While working on bug1393526, it was found out that NFS-Ganesha server intermittently crashes. Further RCA showed that while in the process of exporting a volume, in case if the volume is stopped, NFS-Ganesha server may crash with an assert. The reason being there is a ref leak in the volume export failure cleanup path. So whenever that code-path is taken, server shall crash with below assert
 - assert(export->refcnt  == 1);

Thanks to Pranith for the reproducer.

Version-Release number of selected component (if applicable):
nfs-ganesha-2.4-1

How reproducible:
Often

Steps to Reproduce:

Thanks to Pranith for reproducer. From bug1393526,
>>> 
Here are the steps which exposed this issue:
In terminal 1:
while true; do gluster --mode=script volume stop r2 ; sleep 30 ; gluster --mode=script volume start r2; sleep 30; done

In terminal 2:
while true; do /usr/libexec/ganesha/dbus-send.sh /etc/ganesha on r2; sleep 10 && showmount -e localhost; /usr/libexec/ganesha/dbus-send.sh /etc/ganesha off r2; showmount -e localhost; done

In terminal 3:
watch systemctl status nfs-ganesha
<<<

Actual results:
The server crashes with an assert sometimes.

Expected results:
Server shouldn't crash but handle the export failures gracefully.

Additional info: