x86/speculation/srbds: Do not try to turn mitigation off when not supported

When SRBDS is mitigated by TSX OFF, update_srbds_msr() will still read
and write to MSR_IA32_MCU_OPT_CTRL even when that MSR is not supported
due to not having loaded the appropriate microcode.

Check for X86_FEATURE_SRBDS_CTRL which is set only when the respective
microcode which adds MSR_IA32_MCU_OPT_CTRL is loaded.

Based on a patch by Thadeu Lima de Souza Cascardo <cascardo@canonical.com>.

  [ bp: Massage commit message. ]

Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220401074517.1848264-1-ricardo.canuelo@collabora.com