XArray: Fix xa_alloc when id exceeds max

Specifying a starting ID greater than the maximum ID isn't something
attempted very often, but it should fail.  It was succeeding due to
xas_find_marked() returning the wrong error state, so add tests for
both xa_alloc() and xas_find_marked().

Fixes: b803b42823d0 ("xarray: Add XArray iterators")
Signed-off-by: Matthew Wilcox <willy@infradead.org>
diff --git a/lib/xarray.c b/lib/xarray.c
index bbacca5..5f3f931 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1131,7 +1131,7 @@ void *xas_find_marked(struct xa_state *xas, unsigned long max, xa_mark_t mark)
 		entry = xa_head(xas->xa);
 		xas->xa_node = NULL;
 		if (xas->xa_index > max_index(entry))
-			goto bounds;
+			goto out;
 		if (!xa_is_node(entry)) {
 			if (xa_marked(xas->xa, mark))
 				return entry;
@@ -1180,11 +1180,9 @@ void *xas_find_marked(struct xa_state *xas, unsigned long max, xa_mark_t mark)
 	}
 
 out:
-	if (!max)
+	if (xas->xa_index > max)
 		goto max;
-bounds:
-	xas->xa_node = XAS_BOUNDS;
-	return NULL;
+	return set_bounds(xas);
 max:
 	xas->xa_node = XAS_RESTART;
 	return NULL;