fat: drop lock/unlock super

Removed lock/unlock super. Added a new private s_lock mutex.

Signed-off-by: Marco Stornelli <marco.stornelli@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index bca6d0a..2a18234 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -571,7 +571,7 @@
 	int short_len = 0, fill_len = 0;
 	int ret = 0;
 
-	lock_super(sb);
+	mutex_lock(&sbi->s_lock);
 
 	cpos = filp->f_pos;
 	/* Fake . and .. for the root directory. */
@@ -693,7 +693,7 @@
 	if (unicode)
 		__putname(unicode);
 out:
-	unlock_super(sb);
+	mutex_unlock(&sbi->s_lock);
 	return ret;
 }