Fix wrap issue with 64-bit pwritev2/preadv2

Apparently I missed that Linux does a clever trick to optimize
how we pass in lo/hi offsets for 64-bit, and it encodes the
full offset in just the low part. This caused corruption when
writing with pwritev2 for laver sizes, and bad data for preadv2
for larger sizes.

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