lib/vmalloc: Let arch code pass a value to its setup_mmu() helper

Add an inner __setup_vm() that takes an opaque param and passes said
param along to setup_mmu().  x86 will use the param to configure its page
tables for kernel vs. user so that tests that want to enable SMEP (fault
if kernel executes user page) can do so without resorting to hacks and
without breaking tests that need user pages, i.e. that run user code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622210047.3691840-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>