ifup-ib: fix issue with emtpy MTU settings and failed tests
    
    Because the [ test program does not process arguments in order and
    stop immediately on failure, our test lines would result in the error:
    
    /etc/sysconfig/network-scripts/ifup-ib: line 79: [: too many arguments
    
    when MTU was null.  Fix our test lines to accommodate setting the MTU
    regardless of whether or not an MTU was set in the config file (use
    default values for either connected or unconnected mode, depending
    on which mode the ipoib interface is in)
    
    Signed-off-by: Doug Ledford <dledford@redhat.com>