tipc: set default MTU for UDP media

Currently, all bearers are configured with MTU value same as the
underlying L2 device. However, in case of bearers with media type
UDP, higher throughput is possible with a fixed and higher emulated
MTU value than adapting to the underlying L2 MTU.

In this commit, we introduce a parameter mtu in struct tipc_media
and a default value is set for UDP. A default value of 14k
was determined by experimentation and found to have a higher throughput
than 16k. MTU for UDP bearers are assigned the above set value of
media MTU.

Acked-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: GhantaKrishnamurthy MohanKrishna <mohan.krishna.ghanta.krishnamurthy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>