[PATCH] add nth_page()

Provide a function to get the pageframe number of the nth page at
scatterlist.page.  We cannot just index off scatterlist.page because the
physically-contiguous pages may not be contiguous in mem_map[].
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mm.h

@@ -41,6 +41,8 @@ extern int sysctl_legacy_va_layout;
 #define MM_VM_SIZE(mm)	TASK_SIZE
 #endif
 
+#define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + n)
+
 /*
  * Linux kernel virtual memory manager primitives.
  * The idea being to have a "virtual" mm in the same way