ipack: ipoctal: fix UAF, null-ptr-deref, and use-after-free in cleanup on remove

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

ipack: ipoctal: fix UAF, null-ptr-deref, and use-after-free in cleanup on remove

Three issues arise when the device is removed while a tty session is
still active:

1. UAF of struct ipoctal: the remove callback frees ipoctal via
kfree() while tty ops may still access it. Fix by introducing
kref-based lifetime management — kref is taken in install() when
a tty is opened and released in cleanup() when the tty is finally
destroyed; remove() uses kref_put() instead of kfree().

2. NULL dereference in ipoctal_write_tty(): __ipoctal_remove()
frees xmit_buf via tty_port_free_xmit_buf() while a userspace
process may still hold the tty fd and call write(). Fix by
checking for NULL xmit_buf in ipoctal_write_tty().

3. UAF in ipoctal_cleanup(): ipack_put_carrier(ipoctal->dev)
dereferences ipoctal->dev after the ipack_device has been freed
by ipack_device_del(). Fix by caching ipoctal->carrier_owner
during probe() and calling module_put() on the cached pointer
directly in cleanup(), avoiding any access to ipoctal->dev.

Also introduce a "removed" flag in struct ipoctal, set at the start
of __ipoctal_remove(), and checked in every tty op that accesses
hardware resources (port_activate, write_tty, set_termios, hangup,
shutdown). This prevents page faults when devm_ioremap() regions
are unmapped after remove() returns.