[IA64] xen: Fix return value check in xencomm_vtop()

In case of error, the function follow_page() returns ERR_PTR()
or NULL pointer. The NULL test in the error handling should be
replaced with IS_ERR_OR_NULL().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/xen/xencomm.c

@@ -17,6 +17,7 @@
  */
 
 #include <linux/mm.h>
+#include <linux/err.h>
 
 static unsigned long kernel_virtual_offset;
 static int is_xencomm_initialized;
@@ -98,7 +99,7 @@ xencomm_vtop(unsigned long vaddr)
 
 	/* We assume the page is modified.  */
 	page = follow_page(vma, vaddr, FOLL_WRITE | FOLL_TOUCH);
-	if (!page)
+	if (IS_ERR_OR_NULL(page))
 		return ~0UL;
 
 	return (page_to_pfn(page) << PAGE_SHIFT) | (vaddr & ~PAGE_MASK);