ltq-ptm: Fix unprivileged local user memory read and write

ltq-ptm: Fix unprivileged local user memory read and write

Use the copy_from_user() and copy_to_user() functions for accessing
memory provided by the user in the ptm netdev iotls.
In addition also check for root permission before executing ioctl.

Suggested-by: Stanislav Fort from Aisle Research
Reported-by: Stanislav Fort from Aisle Research
Signed-off-by: Hauke Mehrtens <[email protected]>
(cherry picked from commit 2a76abc5442e3f74d95b4caa9bb57e5488fc132e)