net-shapers: don't free reply skb after genlmsg_reply()

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

net-shapers: don't free reply skb after genlmsg_reply()

genlmsg_reply() hands the reply skb to netlink, and

netlink_unicast() consumes it on all return paths, whether the

skb is queued successfully or freed on an error path.

net_shaper_nl_get_doit() and net_shaper_nl_cap_get_doit()

currently jump to free_msg after genlmsg_reply() fails and call

nlmsg_free(msg), which can hit the same skb twice.

Return the genlmsg_reply() error directly and keep free_msg

only for pre-reply failures.