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/file.c b/fs/f2fs/file.c
index 1f5892f..7f8ca47 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -1591,20 +1591,16 @@
 		return err;
 
 	/* update superblock with uuid */
-	lock_buffer(sbi->raw_super_buf);
 	generate_random_uuid(sbi->raw_super->encrypt_pw_salt);
-	unlock_buffer(sbi->raw_super_buf);
 
 	err = f2fs_commit_super(sbi, false);
-
-	mnt_drop_write_file(filp);
 	if (err) {
 		/* undo new data */
-		lock_buffer(sbi->raw_super_buf);
 		memset(sbi->raw_super->encrypt_pw_salt, 0, 16);
-		unlock_buffer(sbi->raw_super_buf);
+		mnt_drop_write_file(filp);
 		return err;
 	}
+	mnt_drop_write_file(filp);
 got_it:
 	if (copy_to_user((__u8 __user *)arg, sbi->raw_super->encrypt_pw_salt,
 									16))