kernel: Fix s390 absolute memory access for /dev/mem

On s390 the prefix page and absolute zero pages are not correctly
returned when reading /dev/mem. The reason is that the s390 asm/io.h
file includes the asm-generic/io.h file which then defines
xlate_dev_mem_ptr() and therefore overwrites the s390 specific
version that does the correct swap operation for prefix and absolute
zero pages. The problem is a regression that was introduced with git
commit cd248341 (s390/pci: base support).

To fix the problem add "#ifndef xlate_dev_mem_ptr" in asm-generic/io.h
and "#define xlate_dev_mem_ptr" in asm/io.h. This ensures that the
s390 version is used. For completeness also add the "#ifndef"
construct for xlate_dev_kmem_ptr().
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/io.h

@@ -36,6 +36,7 @@ static inline void * phys_to_virt(unsigned long address)
 }
 
 void *xlate_dev_mem_ptr(unsigned long phys);
+#define xlate_dev_mem_ptr xlate_dev_mem_ptr
 void unxlate_dev_mem_ptr(unsigned long phys, void *addr);
 
 /*

include/asm-generic/io.h

@@ -343,8 +343,12 @@ extern void ioport_unmap(void __iomem *p);
 #endif /* CONFIG_GENERIC_IOMAP */
 #endif /* CONFIG_HAS_IOPORT */
 
+#ifndef xlate_dev_kmem_ptr
 #define xlate_dev_kmem_ptr(p)	p
+#endif
+#ifndef xlate_dev_mem_ptr
 #define xlate_dev_mem_ptr(p)	__va(p)
+#endif
 
 #ifdef CONFIG_VIRT_TO_BUS
 #ifndef virt_to_bus