Rework kvm_handle_gfn_range() into an aging-specic helper,
kvm_rmap_age_gfn_range().  In addition to purging a bunch of unnecessary
boilerplate code, this sets the stage for aging rmap SPTEs outside of
mmu_lock.

Note, there's a small functional change, as kvm_test_age_gfn() will now
return immediately if a young SPTE is found, whereas previously KVM would
continue iterating over other levels.

Link: https://lore.kernel.org/r/20240809194335.1726916-15-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>