Fixes a pass-by-value error in FileData.Name()

Fixes a pass-by-value error in FileData.Name() which causes the mutex to be copied, and use that method to retrieve the name of the file from a mem.File.  This really fixes the data race that motivated PR #95. (#96)

I can't explain why moving the lock improves the situation, nor why calling through the accessor Name() instead of locking and reading f.fileData.name is not the same, but go vet indicates that the mutex in fileData was being copied, not preserved.

The reproducing test case upstream is:
check out github.com/google/mtail
make install_deps
go test -race -timeout 1m -v -run TestProcessEvents --count=10000 ./vm

Prior to this change, the test reports a data race 3 times out of 10000, after, 0 times consistently.

mem/file.go

@@ -59,7 +59,9 @@ type FileData struct {
 	modtime time.Time
 }
 
-func (d FileData) Name() string {
+func (d *FileData) Name() string {
+	d.Lock()
+	defer d.Unlock()
 	return d.name
 }
 
@@ -107,9 +109,7 @@ func (f *File) Close() error {
 }
 
 func (f *File) Name() string {
-	f.fileData.Lock()
-	defer f.fileData.Unlock()
-	return f.fileData.name
+	return f.fileData.Name()
 }
 
 func (f *File) Stat() (os.FileInfo, error) {

memmap_test.go

@@ -259,3 +259,26 @@ func TestWriteCloseTime(t *testing.T) {
 		}
 	}
 }
+
+// This test should be run with the race detector on:
+// go test -race -v -timeout 10s -run TestRacingDeleteAndClose
+func TestRacingDeleteAndClose(t *testing.T) {
+	fs := NewMemMapFs()
+	pathname := "testfile"
+	f, err := fs.Create(pathname)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	in := make(chan bool)
+
+	go func() {
+		<-in
+		f.Close()
+	}()
+	go func() {
+		<-in
+		fs.Remove(pathname)
+	}()
+	close(in)
+}