debugfs: fix create mutex racy fops and private data

debugfs: fix create mutex racy fops and private data

commit d3a3b0adad0865c12e39b712ca89efbd0a3a0dbc upstream.

Setting fops and private data outside of the mutex at debugfs file
creation introduces a race where the files can be opened with the wrong
file operations and private data.  It is easy to trigger with a process
waiting on file creation notification.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>