bonding: make mii_status sysfs node consistent

The spew in /proc/net/bonding/bond0 uses netif_carrier_ok() to determine
mii_status, while /sys/class/net/bond0/bonding/mii_status looks at
curr_active_slave, which doesn't actually seem to be set sometimes when
the bond actually is up. A mode 4 bond configured via ifcfg-foo files on a
Red Hat Enterprise Linux system, after boot, comes up clean and
functional, but the sysfs node shows mii_status of down, while proc shows
up. A simple enough fix here seems to be to use the same method for
determining up or down in both places, and I'd opt for the one that seems
to match reality.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <gospo@cumulusnetworks.com>
CC: netdev@vger.kernel.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>