net: psp: do not inherit the Rx association on clone

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

net: psp: do not inherit the Rx association on clone

sk->psp_assoc sits past sk_dontcopy_end, so sock_copy() copies it into
every socket accepted from a listener without taking a reference, while
inet_sock_destruct() puts for every inet socket. psp_twsk_init() does
refcount_inc() for the timewait socket, so a child closing through
TIME_WAIT cancels its own put and leaves the association with one
reference and N timewait sockets holding the same pointer. Closing the
listener frees it, and the timewait timers then put freed memory.

Rejecting the association on a listening socket is not sufficient: a socket
can acquire one while established and then be turned back into a listener,
because tcp_disconnect() leaves sk->psp_assoc in place.

BUG: KASAN: slab-use-after-free in psp_twsk_assoc_free+0x6f/0xf0
Write of size 4 at addr ffff888110f9255c by task swapper/7/0
psp_twsk_assoc_free+0x6f/0xf0
inet_twsk_put+0xda/0x1b0
call_timer_fn+0x53/0x2e0
__run_timers+0x764/0xa80
Freed by task 99:
kfree+0x1a7/0x500
process_one_work+0x7ec/0x1100

An association carries a per-connection SPI and key, so a child must not
inherit the parent's. Clear it on clone.