parisc: Add memory clobber to TLB purges

parisc: Add memory clobber to TLB purges

BugLink: https://bugs.launchpad.net/bugs/1837516
commit 44224bdb99150ad17cf394973b25736cb92c246a upstream.

The pdtlb and pitlb instructions are strongly ordered. The asms invoking
these instructions should be compiler memory barriers to ensure the
compiler doesn't reorder memory operations around these instructions.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
CC: stable@vger.kernel.org # v4.20+
Fixes: 3847dab77421 ("parisc: Add alternative coding infrastructure")
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>