net: fix IP checksum overflow during reduction

When reducing the checksum from a 32-bit or 64-bit intermediate,
we can get an overflow after the first overflow handling step:

0000_8000_8000_ffff
-> 10_ffff
->  1_000f
->    0010

Since we lacked the second step, we got an off-by-one in the checksum.

net/ip.cc

@@ -41,7 +41,8 @@ uint16_t ip_checksum(const void* data, size_t len) {
         len -= 1;
     }
     csum = (csum & 0xffff) + ((csum >> 16) & 0xffff) + ((csum >> 32) & 0xffff) + (csum >> 48);
-    csum += csum >> 16;
+    csum = (csum & 0xffff) + (csum >> 16);
+    csum = (csum & 0xffff) + (csum >> 16);
     return htons(~csum);
 }
 
@@ -144,7 +145,8 @@ void checksummer::sum(const packet& p) {
 }
 
 uint16_t checksummer::get() const {
-    return partial + (partial >> 16);
+    auto tmp = (partial & 0xffff) + (partial >> 16);
+    return tmp + (tmp >> 16);
 }
 
 }