commit edb6f294 upstream.

This affects xen pv guests with sufficiently old versions of xen and
sufficiently new hardware.  On such a system, a guest with a btrfs
root won't even boot.
Signed-off-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reported-by: Michael Marineau <michael.marineau@coreos.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>