RDMA/siw: Reduce memory usage of struct siw_rx_stream

RDMA/siw: Reduce memory usage of struct siw_rx_stream

We can reduce the memory of the struct by move some of it's
member.

Before,

	/* size: 144, cachelines: 3, members: 17 */
	/* sum members: 124, holes: 3, sum holes: 12 */
	/* sum bitfield members: 7 bits (0 bytes) */
	/* padding: 7 */
	/* bit_padding: 1 bits */

After

	/* size: 128, cachelines: 2, members: 17 */
	/* padding: 3 */
	/* bit_padding: 1 bits */

Link: https://lore.kernel.org/r/20231203092655.28102-3-guoqing.jiang@linux.dev
Acked-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>