RDMA/rtrs-srv: Correct the checking of ib_map_mr_sg

We should check with nr_sgt, also the only successful case is that
all sg elements are mapped, so make it explicitly.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20221117101945.6317-4-guoqing.jiang@linux.dev


Signed-off-by: Leon Romanovsky <leon@kernel.org>