Move walk_slot_rmaps() and friends up near for_each_slot_rmap_range() so
that the walkers can be used to handle mmu_notifier invalidations, and so
that similar function has some amount of locality in code.

No functional change intended.

Link: https://lore.kernel.org/r/20240809194335.1726916-11-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>