vhost: fix initialization

Exception handling is executed in the normal path and it will cause
vhost-user init failure.

Fixes: d6983a70e2 ("vhost: check return of pthread calls")

Reported-by: Lei Yao <lei.a.yao@intel.com>
Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

lib/librte_vhost/socket.c

@@ -669,6 +669,9 @@ rte_vhost_driver_register(const char *path, uint64_t flags)
 
 	vhost_user.vsockets[vhost_user.vsocket_cnt++] = vsocket;
 
+	pthread_mutex_unlock(&vhost_user.mutex);
+	return ret;
+
 out_mutex:
 	if (pthread_mutex_destroy(&vsocket->conn_mutex)) {
 		RTE_LOG(ERR, VHOST_CONFIG,