riscv: Return vm_paddr_t in pmap_early_vtophys()

We can't use a u_int to compute the physical address in
pmap_early_vtophys(). Our int is 32-bit, but the physical address is
64-bit. This works fine if everything lives in below 0x100000000, but as
soon as it doesn't this breaks.

MFC after:	1 week
Sponsored by:	Axiado

sys/riscv/riscv/pmap.c

@@ -464,7 +464,7 @@ pmap_early_vtophys(vm_offset_t l1pt, vm_offset_t va)
 {
 	u_int l1_slot, l2_slot;
 	pt_entry_t *l2;
-	u_int ret;
+	vm_paddr_t ret;
 
 	l2 = pmap_early_page_idx(l1pt, va, &l1_slot, &l2_slot);