tracefs: Fix refcount imbalance in start_creating()

In tracefs' start_creating(), we pin the file system to safely access
its root. When we failed to create a file, we unpin the file system via
failed_creating() to release the mount count and eventually the reference
of the singleton vfsmount.

However, when we run into an error during lookup_one_len() when still
in start_creating(), we only release the parent's mutex but not so the
reference on the mount.

F.e., in securityfs_create_file(), after doing simple_pin_fs() when
lookup_one_len() fails there, we infact do simple_release_fs(). This
seems necessary here as well.

Same issue seen in debugfs due to 190afd81e4 ("debugfs: split the
beginning and the end of __create_file() off"), which seemed to got
carried over into tracefs, too. Noticed during code review.

Link: http://lkml.kernel.org/r/68efa86101b778cf7517ed7c6ad573bd69f60ec6.1446672850.git.daniel@iogearbox.net

Fixes: 4282d60689 ("tracefs: Add new tracefs file system")
Cc: stable@vger.kernel.org # 4.1+
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

fs/tracefs/inode.c

@@ -340,8 +340,12 @@ static struct dentry *start_creating(const char *name, struct dentry *parent)
 		dput(dentry);
 		dentry = ERR_PTR(-EEXIST);
 	}
-	if (IS_ERR(dentry))
+
+	if (IS_ERR(dentry)) {
 		mutex_unlock(&parent->d_inode->i_mutex);
+		simple_release_fs(&tracefs_mount, &tracefs_mount_count);
+	}
+
 	return dentry;
 }