tipc: add tipc_bearer_min_mtu to calculate min mtu

stable inclusion
from stable-v5.10.181
commit 097ea78d8ccaf789eeeb154fdfb6afbd274a2a87
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8GJZJ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=097ea78d8ccaf789eeeb154fdfb6afbd274a2a87



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[ 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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>