igb: fix driver reload with VF assigned to guest

commit fa44f2f1 broke reloading of igb, when
VFs are assigned to a guest, in several ways.
1. on module load adapter->vf_data does not get properly allocated,
resulting in a null pointer exception when accessing adapter->vf_data in
igb_reset() on module reload.
 modprobe -r igb ; modprobe igb max_vfs=7
[  215.215837] igb 0000:01:00.1: removed PHC on eth1
[  216.932072] igb 0000:01:00.1: IOV Disabled
[  216.937038] igb 0000:01:00.0: removed PHC on eth0
[  217.127032] igb 0000:01:00.0: Cannot deallocate SR-IOV virtual functions while they are assigned - VFs will not be deallocated
[  217.146178] igb: Intel(R) Gigabit Ethernet Network Driver - version 5.0.5-k
[  217.154050] igb: Copyright (c) 2007-2013 Intel Corporation.
[  217.160688] igb 0000:01:00.0: Enabling SR-IOV VFs using the module parameter is deprecated - please use the pci sysfs interface.
[  217.173703] igb 0000:01:00.0: irq 103 for MSI/MSI-X
[  217.179227] igb 0000:01:00.0: irq 104 for MSI/MSI-X
[  217.184735] igb 0000:01:00.0: irq 105 for MSI/MSI-X
[  217.220082] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
[  217.228846] IP: [<ffffffffa007c5e5>] igb_reset+0xc5/0x4b0 [igb]
[  217.235472] PGD 3607ec067 PUD 36170b067 PMD 0
[  217.240461] Oops: 0002 [#1] SMP
[  217.244085] Modules linked in: igb(+) igbvf mptsas mptscsih mptbase scsi_transport_sas [last unloaded: igb]
[  217.255040] CPU: 4 PID: 4833 Comm: modprobe Not tainted 3.11.0+ #46
[...]
[  217.390007]  [<ffffffffa007fab2>] igb_probe+0x892/0xfd0 [igb]
[  217.396422]  [<ffffffff81470b3e>] local_pci_probe+0x1e/0x40
[  217.402641]  [<ffffffff81472029>] pci_device_probe+0xf9/0x110
[...]
2. A follow up issue, pci_enable_sriov() should only be called if no VFs were
still allocated on module unload. Otherwise pci_enable_sriov() gets called
multiple times in a row rendering the NIC unusable until reset.
3. simply calling igb_enable_sriov() in igb_probe_vfs() is not enough as the
interrupts need to be re-setup. Switching that to igb_pci_enable_sriov().

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Tested-by: Sibai Li <Sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>