x86/spec-ctrl: Fix default calculation of opt_srb_lock

Since this logic was introduced, opt_tsx has become more complicated and
shouldn't be compared to 0 directly.  While there are no buggy logic paths,
the correct expression is !(opt_tsx & 1) but the rtm_disabled boolean is
easier and clearer to use.

Fixes: 8fe24090d9 ("x86/cpuid: Rework HLE and RTM handling")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/spec_ctrl.c

@@ -1245,7 +1245,7 @@ void __init init_speculation_mitigations(void)
          */
         if ( opt_srb_lock == -1 &&
              (caps & (ARCH_CAPS_MDS_NO|ARCH_CAPS_TAA_NO)) == ARCH_CAPS_MDS_NO &&
-             (!cpu_has_hle || ((caps & ARCH_CAPS_TSX_CTRL) && opt_tsx == 0)) )
+             (!cpu_has_hle || ((caps & ARCH_CAPS_TSX_CTRL) && rtm_disabled)) )
             opt_srb_lock = 0;
 
         val &= ~MCU_OPT_CTRL_RNGDS_MITG_DIS;