powerpc/64s: Allow control of RFI flush via debugfs

powerpc/64s: Allow control of RFI flush via debugfs

Expose the state of the RFI flush (enabled/disabled) via debugfs, and
allow it to be enabled/disabled at runtime.

eg: $ cat /sys/kernel/debug/powerpc/rfi_flush
    1
    $ echo 0 > /sys/kernel/debug/powerpc/rfi_flush
    $ cat /sys/kernel/debug/powerpc/rfi_flush
    0

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mauricio: backport: hunk 2: update context lines for end of file]
Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
CVE-2018-3639 (powerpc)

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>