ch4/ofi: fix NIC selection under striping

When striping in enabled, different available NICs should be
used for huge message chunks.

NIC IDs were correctly calculated for the different chunks but
ctx_id was never re-calculated based on the updated NIC IDs.

src/mpid/ch4/netmod/ofi/ofi_huge.c

@@ -98,8 +98,8 @@ static int get_huge_issue_read(MPIR_Request * rreq)
     int issued_chunks = 0;
 
     int nic = 0;
-    int ctx_idx = MPIDI_OFI_get_ctx_index(vci_local, nic);
     while (bytesLeft > 0) {
+        int ctx_idx = MPIDI_OFI_get_ctx_index(vci_local, nic);
         fi_addr_t addr = MPIDI_OFI_comm_to_phys(comm, info->origin_rank, nic, vci_remote);
         uint64_t remote_key = info->rma_keys[nic];