RDMA/rtrs_clt: Alloc less memory with write path fast memory registration

With write path fast memory registration, we need less memory for
each request.

With fast memory registration, we can reduce max_send_sge to save
memory usage.

Also convert the kmalloc_array to kcalloc.

Link: https://lore.kernel.org/r/20210621055340.11789-4-jinpu.wang@ionos.com


Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>