mptcp: defer freeing of cached ext until last moment

access to msk->cached_ext is only legal if the msk is locked or all
concurrent accesses are impossible.

Furthermore, once we start to tear down, we must make sure nothing else
can step in and allocate a new cached ext.

So place this code in the destroy callback where it belongs.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>