powerpc/mm/radix: Use ptep_get_and_clear_full when clearing pte for full mm

powerpc/mm/radix: Use ptep_get_and_clear_full when clearing pte for full mm

BugLink: http://bugs.launchpad.net/bugs/1667116
This helps us to do some optimization for application exit case, where we can
skip the DD1 style pte update sequence.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from linux-next commit f4894b80b1ddfef00d4d2e5c58613ccef358a1b2)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>