bonding: fix slave stuck in BOND_LINK_FAIL state

The bonding miimon logic has a flaw, in that a failure of the
rtnl_trylock can cause a slave to become permanently stuck in
BOND_LINK_FAIL state.

	The sequence of events to cause this is as follows:

	1) bond_miimon_inspect finds that a slave's link is down, and so
calls bond_propose_link_state, setting slave->new_link_state to
BOND_LINK_FAIL, then sets slave->new_link to BOND_LINK_DOWN and returns
non-zero.

	2) In bond_mii_monitor, the rtnl_trylock fails, and the timer is
rescheduled.  No change is committed.

	3) bond_miimon_inspect is called again, but this time the slave
from step 1 has recovered.  slave->new_link is reset to NOCHANGE, and, as
slave->link was never changed, the switch enters the BOND_LINK_UP case,
and does nothing.  The pending BOND_LINK_FAIL state from step 1 remains
pending, as new_link_state is not reset.

	4) The state from step 3 persists until another slave changes link
state and causes bond_miimon_inspect to return non-zero.  At this point,
the BOND_LINK_FAIL state change on the slave from steps 1-3 is committed,
and the slave will remain stuck in BOND_LINK_FAIL state even though it
is actually link up.

	The remedy for this is to initialize new_link_state on each entry
to bond_miimon_inspect, as is already done with new_link.

Fixes: fb9eb899 ("bonding: handle link transition from FAIL to UP correctly")
Reported-by: Alex Sidorenko <alexandre.sidorenko@hpe.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Acked-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>