f2fs: fix defined but not used build warnings

Fix build warnings in f2fs when CONFIG_PROC_FS is not enabled
by marking the unused functions as __maybe_unused.

../fs/f2fs/sysfs.c:519:12: warning: 'segment_info_seq_show' defined but not used [-Wunused-function]
../fs/f2fs/sysfs.c:546:12: warning: 'segment_bits_seq_show' defined but not used [-Wunused-function]
../fs/f2fs/sysfs.c:570:12: warning: 'iostat_info_seq_show' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <yuchao0@huawei.com>
Cc: linux-f2fs-devel@lists.sourceforge.net
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -9,6 +9,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+#include <linux/compiler.h>
 #include <linux/proc_fs.h>
 #include <linux/f2fs_fs.h>
 #include <linux/seq_file.h>
@@ -518,7 +519,8 @@ static struct kobject f2fs_feat = {
 	.kset	= &f2fs_kset,
 };
 
-static int segment_info_seq_show(struct seq_file *seq, void *offset)
+static int __maybe_unused segment_info_seq_show(struct seq_file *seq,
+						void *offset)
 {
 	struct super_block *sb = seq->private;
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
@@ -545,7 +547,8 @@ static int segment_info_seq_show(struct seq_file *seq, void *offset)
 	return 0;
 }
 
-static int segment_bits_seq_show(struct seq_file *seq, void *offset)
+static int __maybe_unused segment_bits_seq_show(struct seq_file *seq,
+						void *offset)
 {
 	struct super_block *sb = seq->private;
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
@@ -569,7 +572,8 @@ static int segment_bits_seq_show(struct seq_file *seq, void *offset)
 	return 0;
 }
 
-static int iostat_info_seq_show(struct seq_file *seq, void *offset)
+static int __maybe_unused iostat_info_seq_show(struct seq_file *seq,
+					       void *offset)
 {
 	struct super_block *sb = seq->private;
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);