KVM: SVM: Massage name and param of helper that merges vmcb01 and vmcb12 MSRPMs

KVM: SVM: Massage name and param of helper that merges vmcb01 and vmcb12 MSRPMs

Rename nested_svm_vmrun_msrpm() to nested_svm_merge_msrpm() to better
capture its role, and opportunistically feed it @vcpu instead of @svm, as
grabbing "svm" only to turn around and grab svm->vcpu is rather silly.

No functional change intended.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20250610225737.156318-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>