tipc: Add support to set and get MTU for UDP bearer

tipc: Add support to set and get MTU for UDP bearer

In this commit we introduce the ability to set and get
MTU for UDP media and bearer.

For set and get properties such as tolerance, window and priority,
we already do:

    $ tipc media set PPROPERTY media MEDIA
    $ tipc media get PPROPERTY media MEDIA

    $ tipc bearer set OPTION media MEDIA ARGS
    $ tipc bearer get [OPTION] media MEDIA ARGS

The same has been extended for MTU, with an exception to support
only media type UDP.

Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: GhantaKrishnamurthy MohanKrishna <mohan.krishna.ghanta.krishnamurthy@ericsson.com>
Signed-off-by: David Ahern <dsahern@gmail.com>