LinuxKPI: implement pci_rescan_bus()

Try to implement pci_rescan_bus(). pci_rescan_method() is already
doing most of the job. We only have to do the count for the return
value again ourselves.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D40122