[PATCH] rcu: Mention rcu_bh in description of rcutorture's torture_type parameter

The comment for rcutorture's torture_type parameter only lists the RCU
variants rcu and srcu, but not rcu_bh; add rcu_bh to the list.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>