6pack: drop redundant locking and refcounting

The TTY layer already serializes line discipline operations with
tty->ldisc_sem, so the extra disc_data_lock and refcnt in 6pack
are unnecessary.

Removing them simplifies the code and also resolves a lockdep warning
reported by syzbot. The warning did not indicate a real deadlock, since
the write-side lock was only taken in process context with hardirqs
disabled.

Reported-by:  <syzbot+5fd749c74105b0e1b302@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68c858b0.050a0220.3c6139.0d1c.GAE@google.com/


Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/20250925051059.26876-1-dqfext@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>