staging: gdm72xx: fix an skb memory leak

The NLMSG_PUT() macro contains a hidden goto that jumps to the
nlmsg_failure label. Since the sk_buff was allocated before the macro,
jumping to the nlmsg_failure label leaks the memory allocated for it.

Calling kfree() before returning would fix it, but is better to avoid
using this error prone macro and use nlmsg_put() instead.

Also, use nlmsg_data() instead of NLMSG_DATA() to check type.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>