KVM: MIPS: Hardcode callbacks to hardware virtualization extensions

Now that KVM no longer supports trap-and-emulate (see commit 45c7e8af
"MIPS: Remove KVM_TE support"), hardcode the MIPS callbacks to the
virtualization callbacks.

Harcoding the callbacks eliminates the technically-unnecessary check on
non-NULL kvm_mips_callbacks in kvm_arch_init().  MIPS has never supported
multiple in-tree modules, i.e. barring an out-of-tree module, where
copying and renaming kvm.ko counts as "out-of-tree", KVM could never
encounter a non-NULL set of callbacks during module init.

The callback check is also subtly broken, as it is not thread safe,
i.e. if there were multiple modules, loading both concurrently would
create a race between checking and setting kvm_mips_callbacks.

Given that out-of-tree shenanigans are not the kernel's responsibility,
hardcode the callbacks to simplify the code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221130230934.1014142-22-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>