Add fd to the data

Add the fd to the SpaData so that we can noth use the fd and the mmapped
memory when we want instead of needing to mmap again in the plugin.
various cleanups

spa/plugins/v4l2/v4l2-source.c

@@ -55,10 +55,9 @@ struct _V4l2Buffer {
   SpaBuffer *outbuf;
   SpaMetaHeader *h;
   bool outstanding;
+  bool allocated;
   struct v4l2_buffer v4l2_buffer;
   V4l2Buffer *next;
-  void *ptr;
-  size_t size;
 };
 
 typedef struct _V4l2Format V4l2Format;