VFS: use synchronize_rcu_expedited() in namespace_unlock()

The synchronize_rcu() in namespace_unlock() is called every time
a filesystem is unmounted. If a great many filesystems are mounted,
this can cause a noticable slow-down in, for example, system shutdown.

The sequence:
mkdir -p /tmp/Mtest/{0..5000}
time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
time umount /tmp/Mtest/*

on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
100 seconds to unmount them.

Boot the same VM with 1 CPU and it takes 18 seconds to mount the
tmpfs filesystems, but only 36 to unmount.

If we change the synchronize_rcu() to synchronize_rcu_expedited()
the umount time on a 4-cpu VM drop to 0.6 seconds

I think this 200-fold speed up is worth the slightly high system
impact of using synchronize_rcu_expedited().

Acked-by: Paul E. McKenney <[e-mail]> (from general rcu perspective)
Signed-off-by: NeilBrown <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/namespace.c

···
 	if (likely(hlist_empty(&head)))
 		return;
 
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 
 	group_pin_kill(&head);
 }