In SDK 15.3.0 and 16.0.0 there exists a naming conflict with the FDS and DFU libraries, where both use the variable named "m_fs", which are globally defined and not static. This causes a compiler error if FDS and DFU code is used together in an application. I recommend either changing the NRF_FSTORAGE_DEF to define the instance as static, or to rename the DFU instance to "m_fs_dfu".