KVM: SVM: Filter out 64-bit exit codes when invoking exit handlers on bare metal

Explicitly filter out 64-bit exit codes when invoking exit handlers, as
svm_exit_handlers[] will never be sized with entries that use bits 63:32.

Processing the non-failing exit code as a 32-bit value will allow tracking
exit_code as a single 64-bit value (which it is, architecturally). This
will also allow hardening KVM against Spectre-like attacks without needing
to do silly things to avoid build failures on 32-bit kernels
(array_index_nospec() rightly asserts that the index fits in an "unsigned
long").

Omit the check when running as a VM, as KVM has historically failed to set
bits 63:32 appropriately when synthesizing VM-Exits, i.e. KVM could get
false positives when running as a VM on an older, broken KVM/kernel. From
a functional perspective, omitting the check is "fine", as any unwanted
collision between e.g. VMEXIT_INVALID and a 32-bit exit code will be
fatal to KVM-on-KVM regardless of what KVM-as-L1 does.

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230211347.4099600-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>