generic/645: Add hint for expected failure with old kernel

generic/645: Add hint for expected failure with old kernel

The following hint is added to reflect that any old kernel
without kernel commit dacfd001eaf2 (“fs/mnt_idmapping.c: Return
-EINVAL when no map is written”) is expected to fail this generic
645 test since without that commit, mount_setattr won’t return
EINVAL when attempting to create an idmapped mount using a user
namespace with no mappings.

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Taylor Jackson <tjackson9431@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>