fault-inject: fix build error when FAULT_INJECTION_CONFIGFS=y and CONFIGFS_FS=m

fault-inject: fix build error when FAULT_INJECTION_CONFIGFS=y and CONFIGFS_FS=m

This fixes a build error when CONFIG_FAULT_INJECTION_CONFIGFS=y and
CONFIG_CONFIGFS_FS=m.

Since the fault-injection library cannot built as a module, avoid building
configfs as a module.

Fixes: 4668c7a2940d ("fault-inject: allow configuration via configfs")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304150025.K0hczLR4-lkp@intel.com/
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>