KVM: x86/mmu: fix determination of max NPT mapping level for private pages

The `if (req_max_level)` test was meant ignore req_max_level if
PG_LEVEL_NONE was returned. Hence, this function should return
max_level instead of the ignored req_max_level.

This is only a latent issue for now, since guest_memfd does not
support large pages.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Message-ID: <20240801173955.1975034-1-ackerleytng@google.com>
Fixes: f32fb328

 ("KVM: x86: Add hook for determining max NPT mapping level")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>