rbtree: add rbtree_postorder_for_each_entry_safe() helper

rbtree: add rbtree_postorder_for_each_entry_safe() helper

Because deletion (of the entire tree) is a relatively common use of the
rbtree_postorder iteration, and because doing it safely means fiddling
with temporary storage, provide a helper to simplify postorder rbtree
iteration.

Signed-off-by: Cody P Schafer <[email protected]>
Reviewed-by: Seth Jennings <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>