checksum: pseudo_header_initial() host byte order

Make the C function pseudo_header_initial() return the checksum in host
byte order. This is to match the other checksum functions and
particularly to make the value suitable as the argument to ipsum().

src/lib/checksum.c

@@ -101,7 +101,7 @@ void checksum_update_incremental_32(uint16_t* checksum_cell,
 // calculates the initial checksum value resulting from
 // the pseudo header.
 // return values:
-// 0x0000 - 0xFFFF : initial checksum (in network order).
+// 0x0000 - 0xFFFF : initial checksum (in host byte order).
 // 0xFFFF0001 : unknown packet (non IPv4/6 or non TCP/UDP)
 // 0xFFFF0002 : bad header
 uint32_t pseudo_header_initial(const int8_t *buf, size_t len)
@@ -146,7 +146,7 @@ uint32_t pseudo_header_initial(const int8_t *buf, size_t len)
     }
     sum = ((sum & 0xffff0000) >> 16) + (sum & 0xffff);
     sum = ((sum & 0xffff0000) >> 16) + (sum & 0xffff);
-    return sum;
+    return ntohs(sum);
   }
   return 0xFFFF0001;
 }