sysfs: fix i_mutex locking in sysfs_get_dentry()

lookup_one_len_kern() should be called with the parent's i_mutex
locked.  Fix it.

Spotted by Eric W. Biederman.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/dir.c

@@ -130,8 +130,10 @@ struct dentry *sysfs_get_dentry(struct sysfs_dirent *sd)
 
 		/* look it up */
 		parent_dentry = dentry;
+		mutex_lock(&parent_dentry->d_inode->i_mutex);
 		dentry = lookup_one_len_kern(cur->s_name, parent_dentry,
 					     strlen(cur->s_name));
+		mutex_unlock(&parent_dentry->d_inode->i_mutex);
 		dput(parent_dentry);
 
 		if (IS_ERR(dentry)) {