alpha/PCI: Add security_locked_down() check to pci_mmap_resource()

In the Linux kernel, the following vulnerability has been resolved: alpha/PCI: Add security_locked_down() check to pci_mmap_resource() Currently, Alpha's pci_mmap_resource() does not check security_locked_down(LOCKDOWN_PCI_ACCESS) before allowing userspace to mmap PCI BARs. The generic version has had this check since commit eb627e17727e ("PCI: Lock down BAR access when the kernel is locked down") to prevent DMA attacks when the kernel is locked down. Add the same check to Alpha's pci_mmap_resource().