nvmf/rdma: Change iovpos of struct nvmf_rdma_request from int to uint32_t

The type of iovcnt of struct spdk_nvmf_request is uint32_t, and so
change the type of iovpos of struct spdk_nvmf_rdma_request from int
to uint32_t.

iovpos of struct spdk_nvmf_rdma_request is only incremented and
accessed. It is not used for comparison.

So to avoid rerunning CI, this fix is appended to the patch series.

Signed-off-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Change-Id: I65fc5dfb7067f6e8f7cb1e555f010b246a72ec32
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/469660


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Alexey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>