mm: de-indent struct page

I found the struct { union { struct { union { struct { } } } } } layout
rather confusing.  Fortunately, there is an easier way to write this.

The innermost union is of four things which are the size of an int, so
the ones which are used by slab/slob/slub can be pulled up two levels to
be in the outermost union with 'counters'.  That leaves us with struct {
union { struct { atomic_t; atomic_t; } } } which has the same layout,
but is easier to read.

Output from the current git version of pahole, diffed with -uw to ignore
the whitespace changes from the indentation:

 	};						/*    16     8 */
 	union {
 		long unsigned int  counters;		/*    24     8 */
-		struct {
-			union {
-				atomic_t _mapcount;	/*    24     4 */
 				unsigned int active;	/*    24     4 */
 				struct {
 					unsigned int inuse:16; /*    24:16  4 */
@@ -21,7 +18,8 @@
 					unsigned int frozen:1; /*    24: 0  4 */
 				};			/*    24     4 */
 				int units;		/*    24     4 */
-			};				/*    24     4 */
+		struct {
+			atomic_t   _mapcount;		/*    24     4 */
 			atomic_t   _refcount;		/*    28     4 */
 		};					/*    24     8 */
 	};						/*    24     8 */

Link: http://lkml.kernel.org/r/20171220155552.15884-3-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

@@ -84,9 +84,15 @@ struct page {
 		 */
 		unsigned counters;
 #endif
-		struct {
+		unsigned int active;		/* SLAB */
+		struct {			/* SLUB */
+			unsigned inuse:16;
+			unsigned objects:15;
+			unsigned frozen:1;
+		};
+		int units;			/* SLOB */
 
-			union {
+		struct {			/* Page cache */
 			/*
 			 * Count of ptes mapped in mms, to show when
 			 * page is mapped & limit reverse map searches.
@@ -98,14 +104,6 @@ struct page {
 			 */
 			atomic_t _mapcount;
 
-				unsigned int active;		/* SLAB */
-				struct {			/* SLUB */
-					unsigned inuse:16;
-					unsigned objects:15;
-					unsigned frozen:1;
-				};
-				int units;			/* SLOB */
-			};
 			/*
 			 * Usage count, *USE WRAPPER FUNCTION* when manual
 			 * accounting. See page_ref.h