wrong size of fields for icmp4 timestamp message

16bits are enough for Id/Seq.

The `len` parameter of `NLMSG_LENGTH`  was size of Netlink header instead of RtNetlink header.

Also make it more clear that the parameter of the macro is the payload size, which is RtNetlink message in this case.

…ompiler (#104)

* python/dnet.pyx: fix incompatible-function-pointer-types for modern compiler

which is error now, see https://bugs.gentoo.org/933360,
clang 19 (maybe earlier) has the same problem too

* python/dnet.pyx: fix -Wincompatible-pointer-types reported by gcc14

./dnet.c:8451:52: error: passing argument 2 of ‘PyObject_AsReadBuffer’ from incompatible pointer type [-Wincompatible-pointer-types]
 8451 |   __pyx_t_1 = (PyObject_AsReadBuffer(__pyx_v_pkt, (&__pyx_v_p), (&__pyx_v_n)) == 0);
      |                                                   ~^~~~~~~~~~~
      |                                                    |
      |                                                    char **
/usr/include/python3.12/abstract.h:370:52: note: expected ‘const void **’ but argument is of type ‘char **’
  370 |                                       const void **buffer,
      |                                       ~~~~~~~~~~~~~^~~~~~
./dnet.c:8451:66: error: passing argument 3 of ‘PyObject_AsReadBuffer’ from incompatible pointer type [-Wincompatible-pointer-types]
 8451 |   __pyx_t_1 = (PyObject_AsReadBuffer(__pyx_v_pkt, (&__pyx_v_p), (&__pyx_v_n)) == 0);
      |                                                                 ~^~~~~~~~~~~
      |                                                                  |
      |                                                                  int *
/usr/include/python3.12/abstract.h:371:51: note: expected ‘Py_ssize_t *’ {aka ‘long int *’} but argument is of type ‘int *’
  371 |                                       Py_ssize_t *buffer_len);
      |                                       ~~~~~~~~~~~~^~~~~~~~~~

Signed-off-by: Z. Liu <[email protected]>