Revert "tipc: do not update mtu if msg_max is too small in mtu negotiation"

This reverts commit 575e84d9 which is
commit 56077b56 upstream.

It breaks the Android kernel abi, so revert it for now.  If it is needed
in the future, it can be brought back in an ABI-safe way.

Bug: 161946584
Change-Id: Ia606733632cba7b98183f09559e6f561653e5733
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>