rculist.h: introduce list_entry_rcu() and list_first_entry_rcu()

I've run into the situation where I need to use list_first_entry with
rcu-guarded list. This patch introduces this.

Also simplify list_for_each_entry_rcu() to use new list_entry_rcu()
instead of list_entry().

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: dipankar@in.ibm.com
LKML-Reference: <20090414153356.GC3999@psychotron.englab.brq.redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>