audit: added capacity and reserve() to nlmsg

There are now two (permitted) ways to add data to netlink message:

1. put_xxx()
2. call nlmsg_reserve() to get a pointer to newly reserved room within the
   original netlink message, then write or memcpy data to that area.

Both of them guarantee adding requested length data do not overflow the
pre-allocated message buffer by checking against its cap field first.

And there may be no need to access nlmsg_len outside nl module, because both
put_xxx() and nlmsg_reserve() have alread did that for us.

Signed-off-by: Shuai Zhang <zs.broccoli@gmail.com>