bonding: improve link-status update in mii-monitoring

The primary issue is that mii-inspect phase updates link-state and
expects changes to be committed during the mii-commit phase. After
the inspect phase if it fails to acquire rtnl-mutex, the commit
phase (bond_mii_commit) doesn't get to run. This partially updated
state stays and makes the internal-state inconsistent.

e.g. setup bond0 => slaves: eth1, eth2
eth1 goes DOWN -> UP
   mii_monitor()
	mii-inspect()
	    bond_set_slave_link_state(eth1, UP, DontNotify)
	rtnl_trylock() <- fails!

Next mii-monitor round
eth1: No change
   mii_monitor()
	mii-inspect()
	    eth1->link == current-status (ethtool_ops->get_link)
	    no-change-detected

End result:
    eth1:
      Link = BOND_LINK_UP
      Speed = 0xfffff  [SpeedUnknown]
      Duplex = 0xff    [DuplexUnknown]

This doesn't always happen but for some unlucky machines in a large set
of machines it creates problems.

The fix for this is to avoid making changes during inspect phase and
postpone them until acquiring the rtnl-mutex / invoking commit phase.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>