Match the change to vop_vptofh in tmpfs proposed by D25077, for fuse.
Instead of duplicating fields of `struct fid` in `struct fuse_fid`, narrow the struct to cover only the unique fields for fuse and assert at compile time that the struct fits in the allotted space. This way we don't have to replicate the offsets of `struct fid` fields, we just use them directly.