lib/test_xarray.c: fix error assumptions on check_xa_multi_store_adv_add()

While testing lib/test_xarray in userspace I've noticed we can fail with:

make -C tools/testing/radix-tree
./tools/testing/radix-tree/xarray

BUG at check_xa_multi_store_adv_add:749
xarray: 0x55905fb21a00x head 0x55905fa1d8e0x flags 0 marks 0 0 0
0: 0x55905fa1d8e0x
xarray: ../../../lib/test_xarray.c:749: check_xa_multi_store_adv_add: Assertion `0' failed.
Aborted

We get a failure with a BUG_ON(), and that is because we actually can
fail due to -ENOMEM, the check in xas_nomem() will fix this for us so
it makes no sense to expect no failure inside the loop. So modify the
check and since this is also useful for instructional purposes clarify
the situation.

The check for XA_BUG_ON(xa, xa_load(xa, index) != p) is already done
at the end of the loop so just remove the bogus on inside the loop.

With this we now pass the test in both kernel and userspace:

In userspace:

./tools/testing/radix-tree/xarray
XArray: 149092856 of 149092856 tests passed

In kernel space:

XArray: 148257077 of 148257077 tests passed

Link: https://lkml.kernel.org/r/20240423192221.301095-3-mcgrof@kernel.org
Fixes: a60cc288 ("test_xarray: add tests for advanced multi-index use")
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/test_xarray.c

@@ -744,15 +744,20 @@ static noinline void check_xa_multi_store_adv_add(struct xarray *xa,
 
 	do {
 		xas_lock_irq(&xas);
-
 		xas_store(&xas, p);
-		XA_BUG_ON(xa, xas_error(&xas));
-		XA_BUG_ON(xa, xa_load(xa, index) != p);
-
 		xas_unlock_irq(&xas);
+		/*
+		 * In our selftest case the only failure we can expect is for
+		 * there not to be enough memory as we're not mimicking the
+		 * entire page cache, so verify that's the only error we can run
+		 * into here. The xas_nomem() which follows will ensure to fix
+		 * that condition for us so to chug on on the loop.
+		 */
+		XA_BUG_ON(xa, xas_error(&xas) && xas_error(&xas) != -ENOMEM);
 	} while (xas_nomem(&xas, GFP_KERNEL));
 
 	XA_BUG_ON(xa, xas_error(&xas));
+	XA_BUG_ON(xa, xa_load(xa, index) != p);
 }
 
 /* mimics page_cache_delete() */