Documentation: Update netlink_mmap.txt

Update netlink_mmap.txt wrt. commit 4682a0358639b29cf
("netlink: Always copy on mmap TX.").

Signed-off-by: Richard Weinberger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/netlink_mmap.txt

···
 TX limitations
 --------------
 
-Kernel processing usually involves validation of the message received by
-user-space, then processing its contents. The kernel must assure that
-userspace is not able to modify the message contents after they have been
-validated. In order to do so, the message is copied from the ring frame
-to an allocated buffer if either of these conditions is false:
-
-- only a single mapping of the ring exists
-- the file descriptor is not shared between processes
-
-This means that for threaded programs, the kernel will fall back to copying.
+As of Jan 2015 the message is always copied from the ring frame to an
+allocated buffer due to unresolved security concerns.
+See commit 4682a0358639b29cf ("netlink: Always copy on mmap TX.").
 
 Example
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