io_uring/rw: use NULL for rw->free_iovec assigment

It's a pointer, don't use 0 for that. sparse throws a warning for that,
as the kernel test robot noticed.

Fixes: d7f1161 ("io_uring/rw: Allocate async data through helper")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412180253.YML3qN4d-lkp@intel.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rw.c

@@ -212,7 +212,7 @@ static void io_rw_async_data_init(void *obj)
 {
 	struct io_async_rw *rw = (struct io_async_rw *)obj;
 
-	rw->free_iovec = 0;
+	rw->free_iovec = NULL;
 	rw->bytes_done = 0;
 }