mm/swapfile.c: unify normal/huge code path in swap_page_trans_huge_swapped()

As suggested by Dave, we should unify the code path for normal and huge
swap support if possible to avoid duplicated code, bugs, etc.  and make
it easier to review code.

In this patch, the normal/huge code path in
swap_page_trans_huge_swapped() is unified, the added and removed lines
are same.  And the binary size is kept almost same when
CONFIG_TRANSPARENT_HUGEPAGE=n.

		 text	   data	    bss	    dec	    hex	filename
base:		24179	   2028	    340	  26547	   67b3	mm/swapfile.o
unified:	24215	   2028	    340	  26583	   67d7	mm/swapfile.o

Link: http://lkml.kernel.org/r/20180720071845.17920-5-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-and-acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -269,7 +269,9 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
 
 static inline bool cluster_is_huge(struct swap_cluster_info *info)
 {
-	return info->flags & CLUSTER_FLAG_HUGE;
+	if (IS_ENABLED(CONFIG_THP_SWAP))
+		return info->flags & CLUSTER_FLAG_HUGE;
+	return false;
 }
 
 static inline void cluster_clear_huge(struct swap_cluster_info *info)
@@ -1424,9 +1426,6 @@ static bool swap_page_trans_huge_swapped(struct swap_info_struct *si,
 	int i;
 	bool ret = false;
 
-	if (!IS_ENABLED(CONFIG_THP_SWAP))
-		return swap_swapcount(si, entry) != 0;
-
 	ci = lock_cluster_or_swap_info(si, offset);
 	if (!ci || !cluster_is_huge(ci)) {
 		if (swap_count(map[roffset]))