arm64: ssbd: Add global mitigation state accessor

arm64: ssbd: Add global mitigation state accessor

BugLink: https://bugs.launchpad.net/bugs/1787993
CVE-2018-3639 (arm64)

commit c32e1736ca03904c03de0e4459a673be194f56fd upstream.

We're about to need the mitigation state in various parts of the
kernel in order to do the right thing for userspace and guests.

Let's expose an accessor that will let other subsystems know
about the state.

Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 8d6907af4583eec62e4eb7f839da3c33d570556e
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git /
linux-4.14.y)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>