XArray: Fix xa_for_each with a single element at 0

The following sequence of calls would result in an infinite loop in
xa_find_after():

	xa_store(xa, 0, x, GFP_KERNEL);
	index = 0;
	xa_for_each(xa, entry, index, ULONG_MAX, XA_PRESENT) { }

xa_find_after() was confusing the situation where we found no entry in
the tree with finding a multiorder entry, so it would look for the
successor entry forever.  Just check for this case explicitly.  Includes
a few new checks in the test suite to be sure this doesn't reappear.

Signed-off-by: Matthew Wilcox <willy@infradead.org>