mptcp: use nlmsg_free instead of kfree_skb

Use nlmsg_free() instead of kfree_skb() in pm_netlink.c.

The SKB's have been created by nlmsg_new(). The proper cleaning way
should then be done with nlmsg_free().

For the moment, nlmsg_free() is simply calling kfree_skb() so we don't
change the behaviour here.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>