x86/speculation/mds: Add 'mitigations=' support for MDS

commit 5c14068f87d04adc73ba3f41c2a303d3c3d1fa12 upstream

Add MDS to the new 'mitigations=' cmdline option.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>