Introduce rculist.h

In linux-next there is a commit ("rcu: split list.h and move rcu-protected
lists into rculist.h") that moved the rcu related list iterators from
list.h to rculist.h.  Add a trivial version of the file now so that
various subsystem trees can start using it now for -next changes and so
reduce the build errors caused by adding uses of the moved functions.

Cc: Franck Bui-Huu <fbuihuu@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@kernel.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>