PCI: Introduce /sys/bus/pci/rescan

This interface allows the user to force a rescan of all PCI buses
in system, and rediscover devices that have been removed earlier.

pci_bus_attrs implementation from Trent Piepho.

Thanks to Vegard Nossum for discovering locking issues with the
sysfs interface.

Cc: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>