vhost: fix deadlock in driver unregister

vhost: fix deadlock in driver unregister

In rte_vhost_driver_unregister(), the connection fd is
removed from the fdset using fdset_try_del(). Call to
this function may fail if the corresponding fd is in
busy state, indicating that event dispatcher is
executing the read or write callback on this fd.
When it happens, rte_vhost_driver_unregister() keeps
trying to remove the fd from the set until it is no
more busy.

This situation is causing a deadlock, because
rte_vhost_driver_unregister() keeps trying to remove
the fd from the set with vhost_user.mutex held, while
the callback executed by the dispatcher,
vhost_user_read_cb(), also takes this mutex at
numerous places.

The fix consists in releasing vhost_user.mutex between
each retry in vhost_driver_unregister().

Fixes: 8b4b949144b8 ("vhost: fix dead lock on closing in server mode")
Cc: stable@dpdk.org
Signed-off-by: Wenjie Sun <findtheonlyway@gmail.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>