To remove duplication with newer kernels, including upstream clients, we should remove as much duplication in the fiemap code as possible. It was added in Lustre before it existed in the upstream kernel, and still has some differences that are not landed upstream, so some care is needed.

Most of ll_fiemap.h could be removed:

• replace struct ll_user_fiemap with struct fiemap in all code (this is already done via #define in ll_fiemap.h)
• replace struct ll_fiemap_extent with struct fiemap_extent in all code
• continue to #define fe_device fe_reserved[0] unless we can get the field assignments changed in upstream
• remove FIEMAP_EXTENT_* constants, except FIEMAP_EXTENT_NET and FIEMAP_EXTENT_NO_DIRECT
• remove FIEMAP_FLAG_* except FIEMAP_EXTENT_DEVICE_ORDER