powerpc/rfi-flush: Move the logic to avoid a redo into the debugfs code

powerpc/rfi-flush: Move the logic to avoid a redo into the debugfs code

rfi_flush_enable() includes a check to see if we're already
enabled (or disabled), and in that case does nothing.

But that means calling setup_rfi_flush() a 2nd time doesn't actually
work, which is a bit confusing.

Move that check into the debugfs code, where it really belongs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 1e2a9fc7496955faacbbed49461d611b704a7505)

CVE-2018-3639 (powerpc)

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