KVM: x86/mmu: rename try_async_pf to kvm_faultin_pfn

KVM: x86/mmu: rename try_async_pf to kvm_faultin_pfn

try_async_pf is a wrong name for this function, since this code
is used when asynchronous page fault is not enabled as well.

This code is based on a patch from Sean Christopherson:
https://lkml.org/lkml/2021/7/19/2970

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210810205251.424103-6-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>