Btrfs: set page->private to the eb

We spend a lot of time looking up extent buffers from pages when we could just
store the pointer to the eb the page is associated with in page->private.  This
patch does just that, and it makes things a little simpler and reduces a bit of
CPU overhead involved with doing metadata IO.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/extent_io.h

@@ -127,6 +127,7 @@ struct extent_buffer {
 	unsigned long map_start;
 	unsigned long map_len;
 	unsigned long bflags;
+	struct extent_io_tree *tree;
 	atomic_t refs;
 	atomic_t pages_reading;
 	struct list_head leak_list;