mm/swap_slots.c: use conditional compilation

For mm/swap_slots.c, use the traditional Linux method of conditional
compilation and linking instead of always compiling it by using #ifdef
CONFIG_SWAP and #endif for the entire source file (excluding header
files).

Link: http://lkml.kernel.org/r/c2a47015-0b5a-d0d9-8bc7-9984c049df20@infradead.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/Makefile

@@ -37,7 +37,7 @@ obj-y			:= filemap.o mempool.o oom_kill.o \
 			   readahead.o swap.o truncate.o vmscan.o shmem.o \
 			   util.o mmzone.o vmstat.o backing-dev.o \
 			   mm_init.o mmu_context.o percpu.o slab_common.o \
-			   compaction.o vmacache.o swap_slots.o \
+			   compaction.o vmacache.o \
 			   interval_tree.o list_lru.o workingset.o \
 			   debug.o $(mmu-y)
 
@@ -55,7 +55,7 @@ ifdef CONFIG_MMU
 endif
 obj-$(CONFIG_HAVE_MEMBLOCK) += memblock.o
 
-obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
+obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o swap_slots.o
 obj-$(CONFIG_FRONTSWAP)	+= frontswap.o
 obj-$(CONFIG_ZSWAP)	+= zswap.o
 obj-$(CONFIG_HAS_DMA)	+= dmapool.o

mm/swap_slots.c

@@ -34,8 +34,6 @@
 #include <linux/mutex.h>
 #include <linux/mm.h>
 
-#ifdef CONFIG_SWAP
-
 static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
 static bool	swap_slot_cache_active;
 bool	swap_slot_cache_enabled;
@@ -356,5 +354,3 @@ repeat:
 
 	return entry;
 }
-
-#endif /* CONFIG_SWAP */