test/perf: optimize the getdents test

* Use mknod instead of open&close to create an empty file.
* Limit a number of files to (1<<16) instead of 100K.

In this case, a test set is (1, 8, 64, 512, 4K, 32K, 64K) instead of (1, 8, 64,
512, 4K, 32K, 98K). I think it is easier to compare results for 32K and 64K
than 32K and 98K. And results for 98K doesn't give us more information than for
54K.

PiperOrigin-RevId: 275552507

test/util/fs_util.cc

@@ -163,6 +163,26 @@ PosixError Chmod(absl::string_view path, int mode) {
   return NoError();
 }
 
+PosixError MknodAt(const FileDescriptor& dfd, absl::string_view path, int mode,
+                   dev_t dev) {
+  int res = mknodat(dfd.get(), std::string(path).c_str(), mode, dev);
+  if (res < 0) {
+    return PosixError(errno, absl::StrCat("mknod ", path));
+  }
+
+  return NoError();
+}
+
+PosixError UnlinkAt(const FileDescriptor& dfd, absl::string_view path,
+                    int flags) {
+  int res = unlinkat(dfd.get(), std::string(path).c_str(), flags);
+  if (res < 0) {
+    return PosixError(errno, absl::StrCat("unlink ", path));
+  }
+
+  return NoError();
+}
+
 PosixError Mkdir(absl::string_view path, int mode) {
   int res = mkdir(std::string(path).c_str(), mode);
   if (res < 0) {

test/util/fs_util.h

@@ -21,6 +21,7 @@
 #include <unistd.h>
 
 #include "absl/strings/string_view.h"
+#include "test/util/file_descriptor.h"
 #include "test/util/posix_error.h"
 
 namespace gvisor {
@@ -44,6 +45,14 @@ PosixError Delete(absl::string_view path);
 // Changes the mode of a file or returns an error.
 PosixError Chmod(absl::string_view path, int mode);
 
+// Create a special or ordinary file.
+PosixError MknodAt(const FileDescriptor& dfd, absl::string_view path, int mode,
+                   dev_t dev);
+
+// Unlink the file.
+PosixError UnlinkAt(const FileDescriptor& dfd, absl::string_view path,
+                    int flags);
+
 // Truncates a file to the given length or returns an error.
 PosixError Truncate(absl::string_view path, int length);