Increase io_submit() coverage

[metan-ucw]

Similarily to the preadv2() the iocb structure now supports RWF_* flags, we should write tests at least for RWF_APPEND and RWF_NOWAIT.

[coolgw]

rewrite RWF_NOWAIT test with following patch
https://patchwork.ozlabs.org/project/ltp/patch/20251022020509.6945-1-wegao@suse.com/