RDMA/rtrs-srv: Refactor the handling of failure case in map_cont_bufs

Let's call unmap_cont_bufs when failure happens, and also only update
mrs_num after everything is settled which means we can remove 'mri'.

Acked-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20221117101945.6317-3-guoqing.jiang@linux.dev


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