mm: remove reference to PG_buddy

PG_buddy doesn't exist any more.  It's called PageBuddy now.

Link: http://lkml.kernel.org/r/20171220155552.15884-9-willy@infradead.orgSigned-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -175,13 +175,13 @@ struct page {
 	};
 
 	union {
-		unsigned long private;		/* Mapping-private opaque data:
-					 	 * usually used for buffer_heads
-						 * if PagePrivate set; used for
-						 * swp_entry_t if PageSwapCache;
-						 * indicates order in the buddy
-						 * system if PG_buddy is set.
-						 */
+		/*
+		 * Mapping-private opaque data:
+		 * Usually used for buffer_heads if PagePrivate
+		 * Used for swp_entry_t if PageSwapCache
+		 * Indicates order in the buddy system if PageBuddy
+		 */
+		unsigned long private;
 #if USE_SPLIT_PTE_PTLOCKS
 #if ALLOC_SPLIT_PTLOCKS
 		spinlock_t *ptl;