Add check for CONFIG_MITIGATION_RFDS

[wryMitts]

Hello, please consider these new options

Intel's hardware vulnurability for Atom cores; Register File Data Sampling.

https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00898.html

Merged by Torvalds

Kconfig

+config MITIGATION_RFDS
+	bool "RFDS Mitigation"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Register File Data Sampling (RFDS) by default.
+	  RFDS is a hardware vulnerability which affects Intel Atom CPUs. It
+	  allows unprivileged speculative access to stale data previously
+	  stored in floating point, vector and integer registers.
+	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
+

Boot flags:

+	reg_file_data_sampling=
+			[X86] Controls mitigation for Register File Data
+			Sampling (RFDS) vulnerability. RFDS is a CPU
+			vulnerability which may allow userspace to infer
+			kernel data values previously stored in floating point
+			registers, vector registers, or integer registers.
+			RFDS only affects Intel Atom processors.
+
+			on:	Turns ON the mitigation.
+			off:	Turns OFF the mitigation.
+
+			This parameter overrides the compile time default set
+			by CONFIG_MITIGATION_RFDS. Mitigation cannot be
+			disabled when other VERW based mitigations (like MDS)
+			are enabled. In order to disable RFDS mitigation all
+			VERW based mitigations need to be disabled.
+
+			For details see:
+			Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst
+

Selected automatically by boot command mitigations=auto per this line: https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/bugs.c#L504

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0e33cf955f07e3991e45109cb3e29fbc9ca51d06

[a13xp0p0v]

Implemented in da9b911

Done, closing.

Thanks!