netlink: Bounds-check struct nlmsgerr creation

In the Linux kernel, the following vulnerability has been resolved:

netlink: Bounds-check struct nlmsgerr creation

In preparation for FORTIFY_SOURCE doing bounds-check on memcpy(),
switch from __nlmsg_put to nlmsg_put(), and explain the bounds check
for dealing with the memcpy() across a composite flexible array struct.
Avoids this future run-time warning:

memcpy: detected field-spanning write (size 32) of single field "&errmsg->msg" at net/netlink/af_netlink.c:2447 (size 16)