rxe: Fix dma.length computation in wr_set_sge_list

wr_set_sge_list() summed the SGE lengths with a loop that never
advanced sg_list:

	while (num_sge--)
		tot_length += sg_list->length;

so tot_length ended up as num_sge * sg_list[0].length instead of the
true sum, and wqe->dma.length / wqe->dma.resid were written with that
wrong value. The per-SGE entries themselves were unaffected because
they are populated by the preceding memcpy().

The kernel rxe driver requires dma.length == sum(sge[i].length) and
enforces it in rxe_mr.c:copy_data(), so a multi-SGE WR posted through
the ibv_qp_ex builder API (ibv_wr_set_sge_list) on rxe completes with
IB_WC_LOC_PROT_ERR once finish_packet()/copy_data() runs off the end
of the SGE list.

The legacy ibv_post_send path (init_send_wqe) is unaffected; it sums
the lengths with an indexed for loop.

Fix by computing the total with an indexed loop, matching the style
already used in rxe_post_one_recv() and init_send_wqe() in this file.

Fixes: 1a894ca10105 ("Providers/rxe: Implement ibv_create_qp_ex verb")
Signed-off-by: Jared Holzman <jholzman@nvidia.com>