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 <paulmck@linux.vnet.ibm.com> (from general rcu perspective)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>