Clarify MTU spacing function

Author: Serock3

talpid-wireguard/src/lib.rs

@@ -1075,7 +1075,8 @@ fn mtu_spacing(mtu_min: u16, mtu_max: u16, step_size: u16) -> Vec<u16> {
     if mtu_min > mtu_max {
         panic!("Invalid MTU detection range: `mtu_min`={mtu_min}, `mtu_max`={mtu_max}.");
     }
-    let in_between = ((mtu_min + 1)..mtu_max).filter(|x| x % step_size == 0);
+    let second_mtu = mtu_min.next_multiple_of(step_size);
+    let in_between = (second_mtu..mtu_max).step_by(step_size as usize);
     let mut ret = Vec::with_capacity(((mtu_max - mtu_min) / 3 + 2) as usize);
     ret.push(mtu_min);
     ret.extend(in_between);