tipc: add tipc_bearer_min_mtu to calculate min mtu

[ Upstream commit 3ae6d66b

 ]

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>
Stable-dep-of: 56077b56

 ("tipc: do not update mtu if msg_max is too small in mtu negotiation")
Signed-off-by: Sasha Levin <sashal@kernel.org>