I think that warning is valid - the host has a 32-bit limit to file sizes (off_t) but it's trying to read a 64-bit offset (in that clause). It's warning you that you won't be able to handle files as large as the field implies.
Can we hide the warning? Probably. Should we? Debatable, as long as we want 64-bit xcoff support in 32-bit filesystems. Otherwise, we'd need to detect off_t overflow somehow, down the slippery slope of reporting the error to the caller...