debugfs: debugfs_lookup() should return NULL if not found

Lots of callers of debugfs_lookup() were just checking NULL to see if
the file/directory was found or not.  By changing this in ff9fb72b
("debugfs: return error values, not NULL") we caused some subsystems to
easily crash.

Fixes: ff9fb72b ("debugfs: return error values, not NULL")
Reported-by:  <syzbot+b382ba6a802a3d242790@syzkaller.appspotmail.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>