Btrfs: Use a mutex in the extent buffer for tree block locking

This replaces the use of the page cache lock bit for locking, which wasn't
suitable for block size < page size and couldn't be used recursively.

The mutexes alone don't fix either problem, but they are the first step.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.h

@@ -90,6 +90,7 @@ struct extent_buffer {
 	int flags;
 	struct list_head leak_list;
 	struct rb_node rb_node;
+	struct mutex mutex;
 };
 
 struct extent_map_tree;