KVM: x86/mmu: Skip rmap operations if rmaps not allocated

If only the TDP MMU is being used to manage the memory mappings for a VM,
then many rmap operations can be skipped as they are guaranteed to be
no-ops. This saves some time which would be spent on the rmap operation.
It also avoids acquiring the MMU lock in write mode for many operations.

This makes it safe to run the VM without rmaps allocated, when only
using the TDP MMU and sets the stage for waiting to allocate the rmaps
until they're needed.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-7-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>