fix memory leak in classify code

If a system has no pci devices the opening of the pci bus directory in
rebuild_irq_db will fail, leading to a memory leak.  Fix it by freeing the list
we create in that function always on exit

Reported-by: Tom Li <biergaizi2009@gmail.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Tom Li <biergaizi2009@gmail.com>