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: d7f11616 ("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>