There were three problems with this API:
1. It took too many arguments; almost all users wanted to iterate over
every element in the array rather than a subset.
2. It required that 'index' be initialised before use, and there's no
realistic way to make GCC catch that.
3. 'index' and 'entry' were the opposite way round from every other
member of the XArray APIs.

So split it into three different APIs:

xa_for_each(xa, index, entry)
xa_for_each_start(xa, index, entry, start)
xa_for_each_marked(xa, index, entry, filter)
Signed-off-by: NMatthew Wilcox <willy@infradead.org>