block: fix parameter not described warning

The build warning:
block/blk-core.c:968: warning: Function parameter or member 'iob'
not described in 'bio_poll'.

Fixes: 5a72e899 ("block: add a struct io_comp_batch argument to fops->iopoll()")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>