f2fs: don't grab super block buffer header all the time

We have already got one copy of valid super block in memory, do not grab
buffer header of super block all the time.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 21048ed..37cf04b 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -715,8 +715,8 @@
 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 valid_super_block;			/* valid super block no */
 	int s_flag;				/* flags for sbi */
 
 	/* for node-related operations */