Input: gscps2 - use RCU for ps2port_list and manage it in open/close

Managing ps2port_list in gscps2_probe() and gscps2_remove() had two
issues:

- in gscps2_remove(), serio_unregister_port() frees the serio port, but
  because the port remained in ps2port_list until later in remove, a
  shared interrupt firing on another CPU could traverse ps2port_list and
  dereference the freed serio port

- ps2port_list additions and deletions in probe/remove raced locklessly
  against list traversals in gscps2_interrupt().

Convert ps2port_list traversal in gscps2_interrupt() to use RCU, and
move list management to gscps2_open() and gscps2_close(). When
serio_unregister_port() runs during device removal, serio_close() is
invoked, cleanly taking the port out of ps2port_list before the serio
structure is destroyed, while maintaining active hardware communication
during child driver disconnect.

Reported-by: sashiko-bot@kernel.org
Assisted-by: LLM
Link: https://patch.msgid.link/20260830-gscps2-v1-2-c733d4cae7f9@gmail.com
Acked-by: Helge Deller <deller@gmx.de>
Tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>