igbvf: clear buffer_info->dma after dma_unmap_single()

igbvf: clear buffer_info->dma after dma_unmap_single()

The driver doesn't clear buffer_info->dma after calling
dma_unmap_single() in all cases. This has been discovered by changing
the mtu twice, which caused the following backtrace.

[   68.569280] WARNING: CPU: 2 PID: 1860 at drivers/iommu/intel-iommu.c:3517 intel_unmap+0x20c/0x220()
[   68.579392] Driver unmaps unmatched page at PFN fffc2a40
[   68.585322] Modules linked in: igbvf ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat kvm_intel kvm igb megs
[   68.599163] CPU: 2 PID: 1860 Comm: ifconfig Not tainted 4.2.0-rc4+ #147
[   68.606543] Hardware name: IBM  -[546025Z]-/00Y7630, BIOS -[VVE134TUS-1.51]- 10/17/2013
[   68.615473]  0000000000000dbd ffff88046441bb08 ffffffff81a5ad0b ffffffff81e2f9ea
[   68.623775]  ffff88046441bb58 ffff88046441bb48 ffffffff81056b55 ffff88047fc583c0
[   68.632075]  0000000000000000 ffff880469a8e600 00000000fffc2a40 ffff880465b32098
[   68.640375] Call Trace:
[   68.643109]  [<ffffffff81a5ad0b>] dump_stack+0x48/0x5d
[   68.648844]  [<ffffffff81056b55>] warn_slowpath_common+0x95/0xe0
[   68.655549]  [<ffffffff81056c56>] warn_slowpath_fmt+0x46/0x70
[   68.661960]  [<ffffffff8158a614>] ? find_iova+0x54/0x90
[   68.667791]  [<ffffffff815988dc>] intel_unmap+0x20c/0x220
[   68.673815]  [<ffffffff8159891e>] intel_unmap_page+0xe/0x10
[   68.680038]  [<ffffffffa0067536>] igbvf_clean_rx_ring+0x96/0x370 [igbvf]
[   68.687516]  [<ffffffffa0067915>] igbvf_down+0x105/0x110 [igbvf]
[   68.694219]  [<ffffffffa0067beb>] igbvf_change_mtu+0x16b/0x180 [igbvf]
[...]

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>