glusterfs: always allocate io_u->engine_data

glusterfs: always allocate io_u->engine_data

When the iodepth is greater then 1 some io_us in fio_gf_io_u_init() can
be fooled into thinking their .engine_data has already been set when it
hasn't. This happens because .index and .engine_data are part of the
same union and .index can be set to non-zero values:

static int init_io_u(struct thread_data *td)
...
        for (i = 0; i < max_units; i++) {
            ...
            io_u->index = i;
            ...
            if (td->io_ops->io_u_init) {
                int ret = td->io_ops->io_u_init(td, io_u);

which means io_u->engine_data != 0 in fio_gf_io_u_init().

Fix the issue by removing the initial check of io_u->engine_data.

Signed-off-by: Simon Gao <simon29rock@gmail.com>