f2fs: add proc entry to monitor current usage of segments

You can monitor valid block counts of whole segments in:
  /proc/fs/f2fs/sdb1/segment_info.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 467d42d..c7620b9 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -350,6 +350,7 @@
 
 struct f2fs_sb_info {
 	struct super_block *sb;			/* pointer to VFS super block */
+	struct proc_dir_entry *s_proc;		/* proc entry */
 	struct buffer_head *raw_super_buf;	/* buffer head of raw sb */
 	struct f2fs_super_block *raw_super;	/* raw super block pointer */
 	int s_dirty;				/* dirty flag for checkpoint */