vt: stabilize tty reference in kbd_keycode with tty_port_tty_get

vt: stabilize tty reference in kbd_keycode with tty_port_tty_get

kbd_keycode() reads vc->port.tty without acquiring a tty reference,
racing against con_shutdown() which clears port.tty under a different
lock. Use tty_port_tty_get()/tty_kref_put() to hold a proper reference
for the duration the tty pointer is needed.

Assisted-by: AISLE:Snapshot
Signed-off-by: Joshua Rogers <linux@joshua.hu>
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/20260731-tty-vt-stuff-v1-1-be99b9da8e30@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>