btrfs: Remove special handling of EXTENT_BUFFER_UNMAPPED while freeing

Now that the whole of btrfs code has been audited for eb reference count
management it's time to remove the hunk in free_extent_buffer that
essentially considered the condition

"eb->ref == 2 && EXTENT_BUFFER_DUMMY"

to equal "eb->ref = 1". Also remove the last location
which takes an extra reference count in alloc_test_extent_buffer.

Signed-off-by: Nikolay Borisov <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/extent_io.c

···
 	check_buffer_tree_ref(eb);
 	set_bit(EXTENT_BUFFER_IN_TREE, &eb->bflags);
 
-	/*
-	 * We will free dummy extent buffer's if they come into
-	 * free_extent_buffer with a ref count of 2, but if we are using this we
-	 * want the buffers to stay in memory until we're done with them, so
-	 * bump the ref count again.
-	 */
-	atomic_inc(&eb->refs);
 	return eb;
 free_eb:
 	btrfs_release_extent_buffer(eb);
···
 	}
 
 	spin_lock(&eb->refs_lock);
-	if (atomic_read(&eb->refs) == 2 &&
-	    test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags))
-		atomic_dec(&eb->refs);
-
 	if (atomic_read(&eb->refs) == 2 &&
 	    test_bit(EXTENT_BUFFER_STALE, &eb->bflags) &&
 	    !extent_buffer_under_io(eb) &&

fs/btrfs/tests/btrfs-tests.c

···
 	/* Will be freed by btrfs_free_fs_roots */
 	if (WARN_ON(test_bit(BTRFS_ROOT_IN_RADIX, &root->state)))
 		return;
-	if (root->node)
+	if (root->node) {
+		/* One for allocate_extent_buffer */
 		free_extent_buffer(root->node);
+		/* One for get_exent_buffer */
+		free_extent_buffer(root->node);
+	}
 	kfree(root);
 }