net: mctp: move expiry timer delete to unhash

Currently, we delete the key expiry timer (in sk->close) before
unhashing the sk. This means that another thread may find the sk through
its presence on the key list, and re-queue the timer.

This change moves the timer deletion to the unhash, after we have made
the key no longer observable, so the timer cannot be re-queued.

Fixes: 7b14e15a

 ("mctp: Implement a timeout for tags")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>