KVM: SVM: Harden exit_code against being used in Spectre-like attacks

Explicitly clamp the exit code used to index KVM's exit handlers to guard
against Spectre-like attacks, mainly to provide consistency between VMX
and SVM (VMX was given the same treatment by commit c926f2f7230b ("KVM:
x86: Protect exit_reason from being used in Spectre-v1/L1TF attacks").

For normal VMs, it's _extremely_ unlikely the exit code could be used to
exploit a speculation vulnerability, as the exit code is set by hardware
and unexpected/unknown exit codes should be quite well bounded (as is/was
the case with VMX).  But with SEV-ES+, the exit code is guest-controlled
as it comes from the GHCB, not from hardware, i.e. an attack from the
guest is at least somewhat plausible.

Irrespective of SEV-ES+, hardening KVM is easy and inexpensive, and such
an attack is theoretically possible.

Link: https://patch.msgid.link/20251230211347.4099600-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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