device: fix alignment on 32-bit machines and test for it

The test previously checked the offset within a substruct, not the
offset within the allocated struct, so this adds the two together.

It then fixes an alignment crash on 32-bit machines.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/alignment_test.go

@@ -61,6 +61,5 @@ func TestDeviceAlignment(t *testing.T) {
 			field.Type.Align(),
 		)
 	}
-
-	checkAlignment(t, "Device.rate.underLoadUntil", unsafe.Offsetof(d.rate.underLoadUntil))
+	checkAlignment(t, "Device.rate.underLoadUntil", unsafe.Offsetof(d.rate)+unsafe.Offsetof(d.rate.underLoadUntil))
 }