PCI: fix reference leak in pci_get_dev_by_id()

Alex Chiang and Matthew Wilcox pointed out that pci_get_dev_by_id() does
not properly decrement the reference on the from pointer if it is
present, like the documentation for the function states it will.

It fixes a pretty bad leak in the hotplug core (we were leaking an
entire struct pci_dev for each function of each offlined card, the first
time around; subsequent onlines/offlines were ok).

Cc: Matthew Wilcox <[e-mail]>
Cc: stable <[e-mail]>
Tested-by: Alex Chiang <[e-mail]>
Acked-by: Alex Chiang <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>

drivers/pci/search.c

···
 			      match_pci_dev_by_id);
 	if (dev)
 		pdev = to_pci_dev(dev);
+	if (from)
+		pci_dev_put(from);
 	return pdev;
 }