iov_iter_fault_in_readable() should do nothing in xarray case

stable inclusion
from linux-4.19.198
commit faf8ab4355ec527f7fd1a4bab8c66f596c8490a8

--------------------------------

commit 0e8f0d67 upstream.

... and actually should just check it's given an iovec-backed iterator
in the first place.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>