add padding to the end of structure

Commit 538a84e1 ("provide padded structure for
table_{key|leaf}_desc API") added explicit padding, if needed,
before every structure member.

This is not enough as if an array of elements are returned
from C++ to python side, the size of structure must be correct.

This patch also adds padding after the last structure field
to make structure size in python side consistent with C++
side. With this patch, I experimented with the pull request
for tcpretrans.py by Matthias Tafelmeier, the type
  struct ipv6_flow_key_t {
      unsigned __int128 saddr;
      unsigned __int128 daddr;
      u64 lport;
      u64 dport;
  }
can be replaced with
  struct ipv6_flow_key_t {
      unsigned __int128 saddr;
      unsigned __int128 daddr;
      u16 lport;
      u16 dport;
  }
where the original type of lport/dport is u16.
Some other ipv6 related data structures can also be simplified.

Signed-off-by: Yonghong Song <yhs@fb.com>