Flush output streams before creating a process to drop caches (#10762)

Small improvement for the `try_drop_l3_cache` feature in cuIO benchmarks. 
Prevents unflushed output from the original process from intermingling with the output from the `popen` process.

Authors:
  - Vukasin Milovanovic (https://github.com/vuule)

Approvers:
  - https://github.com/nvdbaranec
  - Bradley Dice (https://github.com/bdice)

URL: #10762

cpp/benchmarks/io/cuio_common.cpp

@@ -16,6 +16,7 @@
 
 #include <benchmarks/io/cuio_common.hpp>
 
+#include <cstdio>
 #include <fstream>
 #include <numeric>
 #include <string>
@@ -145,6 +146,8 @@ std::vector<cudf::size_type> segments_in_chunk(int num_segments, int num_chunks,
 // Executes the command and returns stderr output
 std::string exec_cmd(std::string_view cmd)
 {
+  // Prevent the output from the command from mixing with the original process' output
+  std::fflush(nullptr);
   // Switch stderr and stdout to only capture stderr
   auto const redirected_cmd = std::string{"( "}.append(cmd).append(" 3>&2 2>&1 1>&3) 2>/dev/null");
   std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(redirected_cmd.c_str(), "r"), pclose);