netlink: Fix potential skb memleak in netlink_ack

Fix coverity issue 'Resource leak'.

We should clean the skb resource if nlmsg_put/append failed.

Fixes: 738136a0

 ("netlink: split up copies in the ack construction")
Signed-off-by: Tao Chen <chentao.kernel@linux.alibaba.com>
Link: https://lore.kernel.org/r/bff442d62c87de6299817fe1897cc5a5694ba9cc.1667638204.git.chentao.kernel@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>