tests: fix idmapped mount_setattr test

The test treated zero as a successful run when it really should treat
non-zero as a successful run. A mount's idmapping can't change once it
has been attached to the filesystem.

Link: https://lore.kernel.org/r/20220203131411.3093040-2-brauner@kernel.org
Fixes: 01eadc8d

 ("tests: add mount_setattr() selftests")
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>