e1000: fix unmap bug

as reported by kerneloops.org

[  121.781161] ------------[ cut here ]------------
[  121.781171] WARNING: at lib/dma-debug.c:793 check_unmap+0x14e/0x577()
[  121.781173] Hardware name: S5520HC
[  121.781177] e1000 0000:0a:00.0: DMA-API: device driver tries to free DMA
memory it has not allocated [device address=0x00000001d688b0fa] [size=1522
bytes]
[  121.781180] Modules linked in: e1000 mdio  dca [last unloaded: ixgbe]
[  121.781187] Pid: 4793, comm: bash Tainted: P 2.6.30-master-06161113 #3
[  121.781190] Call Trace:
[  121.781195]  [<ffffffff8123056f>] ? check_unmap+0x14e/0x577
[  121.781201]  [<ffffffff81057a19>] warn_slowpath_common+0x77/0x8f
[  121.781205]  [<ffffffff81057ae1>] warn_slowpath_fmt+0x9f/0xa1
[  121.781212]  [<ffffffff81477ce2>] ? _spin_lock_irqsave+0x3f/0x49
[  121.781216]  [<ffffffff8122fa97>] ? get_hash_bucket+0x28/0x33
[  121.781220]  [<ffffffff8123056f>] check_unmap+0x14e/0x577
[  121.781225]  [<ffffffff810e4f48>] ? check_bytes_and_report+0x38/0xcb
[  121.781230]  [<ffffffff81230bbf>] debug_dma_unmap_page+0x80/0x92
[  121.781234]  [<ffffffff8122e549>] ? unmap_single+0x1a/0x4e
[  121.781239]  [<ffffffff813901e1>] ? __kfree_skb+0x74/0x78
[  121.781250]  [<ffffffffa00662ef>] pci_unmap_single+0x64/0x6d [e1000]
[  121.781259]  [<ffffffffa0066344>] e1000_clean_rx_ring+0x4c/0xbf [e1000]
[  121.781268]  [<ffffffffa00663df>] e1000_clean_all_rx_rings+0x28/0x36 [e1000]
[  121.781277]  [<ffffffffa0067464>] e1000_down+0x138/0x141 [e1000]
[  121.781286]  [<ffffffffa00681c2>] __e1000_shutdown+0x6b/0x198 [e1000]
[  121.781296]  [<ffffffffa0068405>] e1000_suspend+0x17/0x50 [e1000]
[  121.781301]  [<ffffffff81237665>] pci_legacy_suspend+0x3b/0xbe
[  121.781305]  [<ffffffff81237bc6>] pci_pm_suspend+0x3e/0xf1
[  121.781310]  [<ffffffff812eaf1c>] pm_op+0x57/0xde
[  121.781314]  [<ffffffff812eb444>] dpm_suspend_start+0x31e/0x470
[  121.781319]  [<ffffffff810877da>] suspend_devices_and_enter+0x3e/0x1a2
[  121.781323]  [<ffffffff81087a0f>] enter_state+0xd1/0x127
[  121.781327]  [<ffffffff8108717a>] state_store+0xa7/0xc9
[  121.781332]  [<ffffffff81221843>] kobj_attr_store+0x17/0x19
[  121.781336]  [<ffffffff8113c01e>] sysfs_write_file+0xe5/0x121
[  121.781341]  [<ffffffff810ed165>] vfs_write+0xab/0x105
[  121.781344]  [<ffffffff810ed279>] sys_write+0x47/0x6d
[  121.781349]  [<ffffffff81027aab>] system_call_fastpath+0x16/0x1b
[  121.781352] ---[ end trace 97bacaaac2ed7786 ]---

Fix is to correctly zero out internal ->dma value when unmapping
and make sure never to unmap unless there specifically was a mapping done.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>