arm64: ssbd: Introduce thread flag to control userspace mitigation

In order to allow userspace to be mitigated on demand, let's
introduce a new thread flag that prevents the mitigation from
being turned off when exiting to userspace, and doesn't turn
it on on entry into the kernel (with the assumption that the
mitigation is always enabled in the kernel itself).

This will be used by a prctl interface introduced in a later
patch.

Change-Id: I9583873ac796489eba615e857cb9f21adcef2179
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>
Git-commit: 9dd9614f5476687abbff8d4b12cd08ae70d7c2ad
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>