CVE-2017-5754

BugLink: http://bugs.launchpad.net/bugs/1742772

Rather than assuming a successful return from the hcall will tell us a
valid flush type, if the hcall doesn't select one of the known flush
types use the fallback.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>