Add a typecheck for the endianness conversions

Signed-off-by: Jens Axboe <axboe@fb.com>
client: fix missing pdu->log_offset endianness conversion

Make the iolog sample number uint64_t as well, so we have
the same size across 32 and 64-bit archs.

Signed-off-by: Jens Axboe <axboe@fb.com>