rbd: Use assume_storage_prezeroed when formatting

Instead of passing lazy_itable_init=1 and lazy_journal_init=1 to
mkfs.ext4, pass assume_storage_prezeroed=1 which is
stronger and allows the filesystem to skip inode table zeroing
completely instead of simply doing it lazily.

The support for this flag is checked by trying to format a fake
temporary image with mkfs.ext4 and checking its STDERR.

Closes: #4948
Signed-off-by: Niraj Yadav <niryadav@redhat.com>

internal/util/file/file.go

@@ -52,3 +52,20 @@ func CreateTempFile(prefix, contents string) (*os.File, error) {
 
 	return file, nil
 }
+
+// CreateSpareFile makes `file` a sparse file of size `sizeMB`.
+func CreateSparseFile(file *os.File, sizeMB int64) error {
+	sizeBytes := sizeMB * 1024 * 1024
+
+	// seek to the end of the file.
+	if _, err := file.Seek(sizeBytes-1, 0); err != nil {
+		return fmt.Errorf("failed to seek to the end of the file: %w", err)
+	}
+
+	// write a single byte, effectively making it a sparse file.
+	if _, err := file.Write([]byte{0}); err != nil {
+		return fmt.Errorf("failed to write to the end of the file: %w", err)
+	}
+
+	return nil
+}

internal/util/file/file_test.go

@@ -98,3 +98,45 @@ func TestCreateTempFile_WithLargeContent(t *testing.T) {
 		t.Fatalf("Content mismatch: got %v, want %v", string(readContent), content)
 	}
 }
+
+func TestCreateSparseFile(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		sizeMB  int64
+		wantErr bool
+	}{
+		{"WithValidSize", 10, false},
+		{"WithZeroSize", 0, true},
+		{"WithNegativeSize", -1, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			file, err := os.CreateTemp(t.TempDir(), "test-sparse-")
+			if err != nil {
+				t.Fatalf("Unexpected error: %v", err)
+			}
+
+			err = CreateSparseFile(file, tt.sizeMB)
+			if (err != nil) != tt.wantErr {
+				t.Fatalf("Unexpected error: %v", err)
+			}
+
+			if !tt.wantErr {
+				fileInfo, err := file.Stat()
+				if err != nil {
+					t.Fatalf("Unexpected error: %v", err)
+				}
+
+				expectedSize := tt.sizeMB * 1024 * 1024
+				if fileInfo.Size() != expectedSize {
+					t.Fatalf("Size mismatch: got %v, want %v", fileInfo.Size(), expectedSize)
+				}
+			}
+		})
+	}
+}