fault-inject: clean up debugfs file creation logic

There is no need to check the return value of a debugfs_create_file
call, a caller should never change what they do depending on if debugfs
is working properly or not, so remove the checks, simplifying the logic
in the file a lot.

Also fix up the error check for debugfs_create_dir() which was not
returning NULL for an error, but rather a error pointer.

Cc: linux-kernel@vger.kernel.org
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>