ixgbe: only process one ixgbe_watchdog_task at a time.

Processing multiple ixgbe_watchdog_task calls may cause
the link_up variable and IXGBE_FLAG_NEED_LINK_UPDATE flag
to be set incorrectly.  In the worse case this is causing
the netif_carrier_off to be called inappropriately which
results in an interface that can't be brought up.

Although schedule_work() will only schedule the task if
it is not already on the work queue the WORK_STRUCT_PENDING
bits are cleared just before calling the work function.
This allows WORK_STRUCT_PENDING to be cleared, the work
function to start and meanwhile schedule another task.

This patch adds a mutex to the watchdog task. This bug is
actualized by changing DCB settings or doing extended
cable pull or reset tests.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>