rcu: add comment stating that list_empty() applies to RCU-protected lists
Because list_empty() does not dereference any RCU-protected pointers, and
further does not pass such pointers to the caller (so that the caller
does not dereference them either), it is safe to use list_empty() on
RCU-protected lists. There is no need for a list_empty_rcu(). This
commit adds a comment stating this explicitly.
Requested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>