KVM: x86/mmu: Replace comment with an actual lockdep assertion on mmu_lock

Assert that mmu_lock is held for write in __walk_slot_rmaps() instead of
hoping the function comment will magically prevent introducing bugs.

Signed-off-by: Ben Gardon <bgardon@google.com>
Link: https://lore.kernel.org/r/20230202182809.1929122-3-bgardon@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>