mptcp: fix race on unaccepted mptcp sockets

In the Linux kernel, the following vulnerability has been resolved:

mptcp: fix race on unaccepted mptcp sockets

When the listener socket owning the relevant request is closed,

it frees the unaccepted subflows and that causes later deletion

of the paired MPTCP sockets.

The mptcp socket's worker can run in the time interval between such delete

operations. When that happens, any access to msk->first will cause an UaF

access, as the subflow cleanup did not cleared such field in the mptcp

socket.

Address the issue explicitly traversing the listener socket accept

queue at close time and performing the needed cleanup on the pending

msk.

Note that the locking is a bit tricky, as we need to acquire the msk

socket lock, while still owning the subflow socket one.