s390/pci: Fix s390_pci_mmio_write syscall error return without MIO

On a machine without PCI memory-I/O (MIO) support or when running with
pci=nomio the s390 specific PCI MMIO write syscall checks if the MMIO
cookie is above ZPCI_IOMAP_ADDR_BASE as a sanity check before even
trying to perform the MMIO. If this check fails the return value was
left unchanged and thus 0 from prior operations falsely indicating
success. This could potentially confuse user-space into falsely
believing the MMIO, on a mapping not valid for MMIO was successful.

Fix this by setting the return value to -EFAULT prior to the check
following the same pattern as elsewhere in the same function.

Cc: stable@vger.kernel.org
Reviewed-by: Julian Ruess <julianr@linux.ibm.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Fixes: a67a88b0b8 ("s390/pci: remove races against pte updates")
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/pci/pci_mmio.c

@@ -188,6 +188,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
 			goto out_unlock_mmap;
 	}
 
+	ret = -EFAULT;
 	io_addr = (void __iomem *)((args.pfn << PAGE_SHIFT) |
 			(mmio_addr & ~PAGE_MASK));