r8169: balance pci_map / pci_unmap pair

The leak hurts with swiotlb and jumbo frames.

Fix http://bugzilla.kernel.org/show_bug.cgi?id=9468

.

Heavily hinted by Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Alistair John Strachan <alistair@devzero.co.uk>
Tested-by: Timothy J Fontaine <tjfontaine@atxconsulting.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>