Adding the write bit and RC bits to pte permissions does not require a
pte clear and flush. There should not be other bits changed here,
because restricting access or changing the PFN must have already
invalidated any existing ptes (otherwise the race is already lost).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>