igb: Fix null pointer dereference

The max_vfs= option has always been self limiting to the number of VFs
supported by the device.  fa44f2f1 added SR-IOV configuration via
sysfs, but in the process broke this self correction factor.  The
failing path is:

igb_probe
  igb_sw_init
    if (max_vfs > 7) {
        adapter->vfs_allocated_count = 7;
    ...
    igb_probe_vfs
    igb_enable_sriov(, max_vfs)
      if (num_vfs > 7) {
        err = -EPERM;
        ...

This leaves vfs_allocated_count = 7 and vf_data = NULL, so we bomb out
when igb_probe finally calls igb_reset.  It seems like a really bad
idea, and somewhat pointless, to set vfs_allocated_count separate from
vf_data, but limiting max_vfs is enough to avoid the null pointer.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -2652,7 +2652,7 @@ static int igb_sw_init(struct igb_adapter *adapter)
 		if (max_vfs > 7) {
 			dev_warn(&pdev->dev,
 				 "Maximum of 7 VFs per PF, using max\n");
-			adapter->vfs_allocated_count = 7;
+			max_vfs = adapter->vfs_allocated_count = 7;
 		} else
 			adapter->vfs_allocated_count = max_vfs;
 		if (adapter->vfs_allocated_count)