Documentation: Update netlink_mmap.txt

TX limitations

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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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