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

This reverts commit 2bd4ff4f which is
commit 56077b56 upstream.

It breaks the Android kernel ABI and is not needed for Android devices,
so it is safe to revert for now.  If it is determined that it is needed
in the future, it can be brought back in an abi-preserving way.

Bug: 161946584
Change-Id: I5fc048bf94f78a38691e3b27cf225536200bcd49
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>