KVM: x86: Allow, don't ignore, same-value writes to immutable MSRs

When handling userspace writes to immutable feature MSRs for a vCPU that
has already run, fall through into the normal code to set the MSR instead
of immediately returning '0'.  I.e. allow such writes, instead of ignoring
such writes.  This fixes a bug where KVM incorrectly allows writes to the
VMX MSRs that enumerate which CR{0,4} can be set, but only if the vCPU has
already run.

The intent of returning '0' and thus ignoring the write, was to avoid any
side effects, e.g. refreshing the PMU and thus doing weird things with
perf events while the vCPU is running.  That approach sounds nice in
theory, but in practice it makes it all but impossible to maintain a sane
ABI, e.g. all VMX MSRs return -EBUSY if the CPU is post-VMXON, and the VMX
MSRs for fixed-1 CR bits are never writable, etc.

As for refreshing the PMU, kvm_set_msr_common() explicitly skips the PMU
refresh if MSR_IA32_PERF_CAPABILITIES is being written with the current
value, specifically to avoid unwanted side effects.  And if necessary,
adding similar logic for other MSRs is not difficult.

Fixes: 0094f62c

 ("KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN")
Reported-by: Jim Mattson <jmattson@google.com>
Cc: Raghavendra Rao Ananta <rananta@google.com>
Link: https://lore.kernel.org/r/20240408231500.1388122-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>