Bluetooth: 6lowpan: Pin module for 6lowpan_control debugfs file

An open debugfs file retains lowpan_control_fops in the debugfs proxy
state. However, lowpan_control_fops has no .owner set to THIS_MODULE.
As a result, full_proxy_open_regular() calls fops_get() which succeeds
without taking a reference to the bluetooth_6lowpan module.

With CONFIG_BT_6LOWPAN=m, a userspace process can keep the debugfs file
descriptor for 6lowpan_control open across module unload
(delete_module / rmmod bluetooth_6lowpan). When the process subsequently
closes the file descriptor, full_proxy_release() accesses the already
freed and unmapped lowpan_control_fops table, causing a kernel page fault
or oops.

The issue was discovered via manual code audit of
net/bluetooth/6lowpan.c.

Set .owner = THIS_MODULE on lowpan_control_fops so that debugfs pins
the module until all open file descriptors have finished.

Fixes: 6b8d4a6a0314 ("Bluetooth: 6LoWPAN: Use connected oriented channel instead of fixed one")
Cc: stable@vger.kernel.org
Signed-off-by: Bui Viet Dung <dungvn2345@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>