io_submit01: make sure uninitialized iocb fails

Make sure, that uninitialized iocb test fails. Even if all
"random" data in iocb struct passes all checks, make sure
it eventually fails in opcode check.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
io_submit01: make sure that ctx is invalid in each loop

We need to make "ctx" invalid in each loop, because
this variable gets initialized to valid io_context_t
in subsequent test.

Signed-off-by: Jan Stancek <jstancek@redhat.com>