x86/mce/amd: Check SMCA feature bit before accessing SMCA MSRs

People do effort to inject MCEs into guests in order to simulate/test
handling of hardware errors. The real use case behind it is testing the
handling of SIGBUS which the memory failure code sends to the process.

If that process is QEMU, instead of killing the whole guest, the MCE can
be injected into the guest kernel so that latter can attempt proper
handling and kill the user *process*  in the guest, instead, which
caused the MCE. The assumption being here that the whole injection flow
can supply enough information that the guest kernel can pinpoint the
right process. But that's a different topic...

Regardless of virtualization or not, access to SMCA-specific registers
like MCA_DESTAT should only be done after having checked the smca
feature bit. And there are AMD machines like Bulldozer (the one before
Zen1) which do support deferred errors but are not SMCA machines.

Therefore, properly check the feature bit before accessing related MSRs.

  [ bp: Rewrite commit message. ]

Fixes: 7cb735d7c0cb ("x86/mce: Unify AMD DFR handler with MCA Polling")
Signed-off-by: William Roche <william.roche@oracle.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Yazen Ghannam <yazen.ghannam@amd.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20260218163025.1316501-1-william.roche@oracle.com

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