igb: Fix Tx hangs seen when loading igb with max_vfs > 7.

Check the value of max_vfs at the time of assignment of vfs_allocated_count.

The previous check in igb_probe_vfs was too late as by that time the rx/tx
rings were initialized with the wrong offset.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>