isdn: Disable IIOCDBGVAR

[ Upstream commit 5e22002aa8809e2efab2da95855f73f63e14a36c ]

It was possible to directly leak the kernel address where the isdn_dev
structure pointer was stored. This is a kernel ASLR bypass for anyone
with access to the ioctl. The code had been present since the beginning
of git history, though this shouldn't ever be needed for normal operation,
therefore remove it.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>