<linux/list.h>: Introduce the list_for_each_reverse() method

The list_head counterpart of list_for_each_entry_reverse() was missing,
add it to complete the list handling APIs in <linux/list.h>.

[ This new API is also relied on by a WIP scheduler patch, so this
  variant is not a theoretical possibility only. ]

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org