KVM: x86: allow KVM_REQ_GET_NESTED_STATE_PAGES outside guest mode for VMX

VMX also uses KVM_REQ_GET_NESTED_STATE_PAGES for the Hyper-V eVMCS,
which may need to be loaded outside guest mode.  Therefore we cannot
WARN in that case.

However, that part of nested_get_vmcs12_pages is _not_ needed at
vmentry time.  Split it out of KVM_REQ_GET_NESTED_STATE_PAGES handling,
so that both vmentry and migration (and in the latter case, independent
of is_guest_mode) do the parts that are needed.

Cc: <stable@vger.kernel.org> # 5.10.x: f2c7ef3b

: KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
Cc: <stable@vger.kernel.org> # 5.10.x
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>