drivers: staging: zcache: fix compile error

Because 'ramster_debugfs_init' is not defined if !CONFIG_DEBUG_FS, there is
compile error:

$ make drivers/staging/zcache/
staging/zcache/ramster/ramster.c: In function ‘ramster_init’:
staging/zcache/ramster/ramster.c:981:2: error: implicit declaration of
function ‘ramster_debugfs_init’ [-Werror=implicit-function-declaration]

This patch fix it and reduce some #ifdef CONFIG_DEBUG_FS in .c files the same
way.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/ramster/ramster.c

@@ -134,6 +134,11 @@ static int __init ramster_debugfs_init(void)
 }
 #undef	zdebugfs
 #undef	zdfs64
+#else
+static inline int ramster_debugfs_init(void)
+{
+	return 0;
+}
 #endif
 
 static LIST_HEAD(ramster_rem_op_list);

drivers/staging/zcache/zbud.c

@@ -342,6 +342,11 @@ static int zbud_debugfs_init(void)
 }
 #undef	zdfs
 #undef	zdfs64
+#else
+static inline int zbud_debugfs_init(void)
+{
+	return 0;
+}
 #endif
 
 /* protects the buddied list and all unbuddied lists */
@@ -1051,9 +1056,7 @@ void zbud_init(void)
 {
 	int i;
 
-#ifdef CONFIG_DEBUG_FS
 	zbud_debugfs_init();
-#endif
 	BUG_ON((sizeof(struct tmem_handle) * 2 > CHUNK_SIZE));
 	BUG_ON(sizeof(struct zbudpage) > sizeof(struct page));
 	for (i = 0; i < NCHUNKS; i++) {

drivers/staging/zcache/zcache-main.c

@@ -1733,9 +1733,7 @@ static int __init zcache_init(void)
 		namestr = "ramster";
 		ramster_register_pamops(&zcache_pamops);
 	}
-#ifdef CONFIG_DEBUG_FS
 	zcache_debugfs_init();
-#endif
 	if (zcache_enabled) {
 		unsigned int cpu;