mm: use 'const char *' insted of 'char *' for reason in dump_page()

I tried to use 'dump_page(page, __func__)' for debugging, but it triggers
warning:

  warning: passing argument 2 of `dump_page' discards `const' qualifier from pointer target type [enabled by default]

Let's convert 'reason' to 'const char *' in dump_page() and friends: we
shouldn't modify it anyway.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index 5042c03..2d57efa 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -3,8 +3,8 @@
 
 struct page;
 
-extern void dump_page(struct page *page, char *reason);
-extern void dump_page_badflags(struct page *page, char *reason,
+extern void dump_page(struct page *page, const char *reason);
+extern void dump_page_badflags(struct page *page, const char *reason,
 			       unsigned long badflags);
 
 #ifdef CONFIG_DEBUG_VM