Blender Git Loki

Cleanup: dna_genfile API for accessing struct member offsets

- Rename `find_elem` to `elem_offset` (matching `elem_exists`).

- Remove unused `SDNA_StructMember` return argument.

- Return an offset instead of a pointer which was being converted
back into an offset by one caller,
in this case there was no way to tell the difference between
and element that doesn't exist and a struct member
at the start of the array.

Resolves UBSan warning raised in T81340.