V4L/DVB (7121): Renames videobuf lock to vb_lock

This helps to identify where vb_lock is being used, and find missusages of the
locks.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
diff --git a/include/media/videobuf-core.h b/include/media/videobuf-core.h
index 97f14d4..9903394 100644
--- a/include/media/videobuf-core.h
+++ b/include/media/videobuf-core.h
@@ -149,7 +149,7 @@
 };
 
 struct videobuf_queue {
-	struct mutex               lock;
+	struct mutex               vb_lock;
 	spinlock_t                 *irqlock;
 	void			   *dev; /* on pci, points to struct pci_dev */