tipc: add tipc_bearer_min_mtu to calculate min mtu
As different media may requires different min mtu, and even the
same media with different net family requires different min mtu,
add tipc_bearer_min_mtu() to calculate min mtu accordingly.
This API will be used to check the new mtu when doing the link
mtu negotiation in the next patch.
Signed-off-by: Xin Long <lucien.xin@gmail.com> Acked-by: Jon Maloy <jmaloy@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>