mm: move swp_entry_t definition to include/linux/mm_types.h

swp_entry_t being defined in include/linux/swap.h instead of
include/linux/mm_types.h causes cyclic include dependency later when
include/linux/page_cgroup.h is included from writeback path.  Move the
definition to include/linux/mm_types.h.

While at it, reformat the comment above it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -534,4 +534,12 @@ enum tlb_flush_reason {
 	NR_TLB_FLUSH_REASONS,
 };
 
+ /*
+  * A swap entry has to fit into a "unsigned long", as the entry is hidden
+  * in the "index" field of the swapper address space.
+  */
+typedef struct {
+	unsigned long val;
+} swp_entry_t;
+
 #endif /* _LINUX_MM_TYPES_H */

include/linux/swap.h

@@ -102,14 +102,6 @@ union swap_header {
 	} info;
 };
 
- /* A swap entry has to fit into a "unsigned long", as
-  * the entry is hidden in the "index" field of the
-  * swapper address space.
-  */
-typedef struct {
-	unsigned long val;
-} swp_entry_t;
-
 /*
  * current->reclaim_state points to one of these when a task is running
  * memory reclaim