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.

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>

arch/arm64/kernel/entry.S

@@ -147,6 +147,8 @@ alternative_cb	arm64_enable_wa2_handling
 alternative_cb_end
 	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
 	cbz	\tmp2, \targ
+	ldr	\tmp2, [tsk, #TSK_TI_FLAGS]
+	tbnz	\tmp2, #TIF_SSBD, \targ
 	mov	w0, #ARM_SMCCC_ARCH_WORKAROUND_2
 	mov	w1, #\state
 alternative_cb	arm64_update_smccc_conduit