io_uring/rw: cast rw->flags assignment to rwf_t

io_uring/rw: cast rw->flags assignment to rwf_t

kernel test robot reports that a recent change of the sqe->rw_flags
field throws a sparse warning on 32-bit archs:

>> io_uring/rw.c:291:19: sparse: sparse: incorrect type in assignment (different base types) @@     expected restricted __kernel_rwf_t [usertype] flags @@     got unsigned int @@
   io_uring/rw.c:291:19: sparse:     expected restricted __kernel_rwf_t [usertype] flags
   io_uring/rw.c:291:19: sparse:     got unsigned int

Force cast it to rwf_t to silence that new sparse warning.

Fixes: cf73d9970ea4 ("io_uring: don't use int for ABI")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507032211.PwSNPNSP-lkp@intel.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>