examples/io_uring-test: don't error on shorter files
examples/io_uring-test: don't error on shorter files

The example currently assumes the file is at least 4*4096 bytes,
and errors when we get 0 reads on EOF. Fix this up.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing