Apply a small optimization to pmap_remove_l3_range(). Specifically, hoist a
PHYS_TO_VM_PAGE() operation that always returns the same vm_page_t out of
the loop. (Since arm64 is configured as VM_PHYSSEG_SPARSE, the
implementation of PHYS_TO_VM_PAGE() is more costly than that of
VM_PHYSSEG_DENSE platforms, like amd64.)

MFC after: 1 week