RDMA/rtrs-srv: Pass the correct number of entries for dma mapped SGL

ib_dma_map_sg() augments the SGL into a 'dma mapped SGL'. This process
may change the number of entries and the lengths of each entry.

Code that touches dma_address is iterating over the 'dma mapped SGL'
and must use dma_nents which returned from ib_dma_map_sg().

We should use the return count from ib_dma_map_sg for futher usage.

Fixes: 9cb83748

 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20220818105355.110344-4-haris.iqbal@ionos.com
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: Aleksei Marov <aleksei.marov@ionos.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>