engine/rdmaio: fix io_u initialization

engine/rdmaio: fix io_u initialization

Currenly rdmaio engine fataly broken.
We fill io_u buffer inside engine->init() phase, but at this point td->io_u_freelist is empty,
so initialization code does nothing, so io_u->engine_data will be unitialized,
later this result in null pointer dereferent in fio_rdmaio_prep()

This patch moves io_u initialization to post_init() callback