fuse: dump WRITE argument in debug logs

This provides feedback to file system authors about what we receive
from the kernel. This can help figure out which system is to blame for
data corruption.

Change-Id: I7cd7f340bf705c9972c95cfc6d3d8612f1ce8b34

fuse/request.go

@@ -88,8 +88,19 @@ func (r *request) InputDebug() string {
 		names = fmt.Sprintf("%q", r.filenames)
 	}
 
-	if len(r.arg) > 0 {
-		names += fmt.Sprintf(" %db", len(r.arg))
+	if l := len(r.arg); l > 0 {
+		data := ""
+		if len(r.filenames) == 0 {
+			dots := ""
+			if l > 8 {
+				l = 8
+				dots = "..."
+			}
+
+			data = fmt.Sprintf("%q%s", r.arg[:l], dots)
+		}
+
+		names += fmt.Sprintf("%s %db", data, len(r.arg))
 	}
 
 	return fmt.Sprintf("rx %d: %s i%d %s%s",