usb: fix number of mapped SG DMA entries

usb: fix number of mapped SG DMA entries

Add a new field num_mapped_sgs to struct urb so that we have a place to
store the number of mapped entries and can also retain the original
value of entries in num_sgs.  Previously, usb_hcd_map_urb_for_dma()
would overwrite this with the number of mapped entries, which would
break dma_unmap_sg() because it requires the original number of entries.

This fixes warnings like the following when using USB storage devices:
 ------------[ cut here ]------------
 WARNING: at lib/dma-debug.c:902 check_unmap+0x4e4/0x695()
 ehci_hcd 0000:00:12.2: DMA-API: device driver frees DMA sg list with different entry count [map count=4] [unmap count=1]
 Modules linked in: ohci_hcd ehci_hcd
 Pid: 0, comm: kworker/0:1 Not tainted 3.2.0-rc2+ #319
 Call Trace:
  <IRQ>  [<ffffffff81036d3b>] warn_slowpath_common+0x80/0x98
  [<ffffffff81036de7>] warn_slowpath_fmt+0x41/0x43
  [<ffffffff811fa5ae>] check_unmap+0x4e4/0x695
  [<ffffffff8105e92c>] ? trace_hardirqs_off+0xd/0xf
  [<ffffffff8147208b>] ? _raw_spin_unlock_irqrestore+0x33/0x50
  [<ffffffff811fa84a>] debug_dma_unmap_sg+0xeb/0x117
  [<ffffffff8137b02f>] usb_hcd_unmap_urb_for_dma+0x71/0x188
  [<ffffffff8137b166>] unmap_urb_for_dma+0x20/0x22
  [<ffffffff8137b1c5>] usb_hcd_giveback_urb+0x5d/0xc0
  [<ffffffffa0000d02>] ehci_urb_done+0xf7/0x10c [ehci_hcd]
  [<ffffffffa0001140>] qh_completions+0x429/0x4bd [ehci_hcd]
  [<ffffffffa000340a>] ehci_work+0x95/0x9c0 [ehci_hcd]
  ...
 ---[ end trace f29ac88a5a48c580 ]---
 Mapped at:
  [<ffffffff811faac4>] debug_dma_map_sg+0x45/0x139
  [<ffffffff8137bc0b>] usb_hcd_map_urb_for_dma+0x22e/0x478
  [<ffffffff8137c494>] usb_hcd_submit_urb+0x63f/0x6fa
  [<ffffffff8137d01c>] usb_submit_urb+0x2c7/0x2de
  [<ffffffff8137dcd4>] usb_sg_wait+0x55/0x161

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>