ptp: fix use-after-free in extts_fifo_show() reading tsevqs unlocked

extts_fifo_show() walks ptp->tsevqs with list_count_nodes() and then
list_first_entry() without holding ptp->tsevqs_lock, the spinlock that
"protects tsevqs from concurrent access". Every other accessor of the
list honours it: ptp_open() (list_add_tail), ptp_release() (list_del
followed by kfree of the per-fd queue), ptp_clock_release() and
ptp_clock_event() all take tsevqs_lock. extts_fifo_show() is the only
one that does not.

A per-fd timestamp queue is appended to tsevqs on open("/dev/ptpN") and
removed and freed on close(), both under tsevqs_lock:

	scoped_guard(spinlock_irq, &ptp->tsevqs_lock)
		list_del(&queue->qlist);
	bitmap_free(queue->mask);
	kfree(queue);

So a read of /sys/class/ptp/ptpN/fifo running concurrently with a
close() of a /dev/ptpN fd can have list_count_nodes() walk into the
queue that close() is removing and freeing, dereferencing freed memory
(use-after-free) or a poisoned list pointer.

Take tsevqs_lock around the list walk. The fifo only ever draws from the
first queue, which is the registration-time queue created in
ptp_clock_register() and freed only in ptp_clock_release(); that cannot
run while the sysfs attribute is being read, so the dequeue from
queue->buf under queue->lock may stay outside tsevqs_lock and the lock
order is unchanged.

Fixes: d26ab5a35ad9 ("ptp: Replace timestamp event queue with linked list")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
Link: https://patch.msgid.link/20261026100535.68114-1-yuz08559@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>