connector - documentation: simplify netlink message length assignment

Use the precalculated size instead of obfuscating the message length
calculation by first subtracting the netlink header length from size
and then use the NLMSG_LENGTH() macro to add it back again.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>