btrfs: unify extent buffer allocation api

Make the extent buffer allocation interface consistent.  Cloned eb will
set a valid fs_info.  For dummy eb, we can drop the length parameter and
set it from fs_info.

The built-in sanity checks may pass a NULL fs_info that's queried for
nodesize, but we know it's 4096.

Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/tests/extent-buffer-tests.c

@@ -53,7 +53,7 @@ static int test_btrfs_split_item(void)
 		return -ENOMEM;
 	}
 
-	path->nodes[0] = eb = alloc_dummy_extent_buffer(0, 4096);
+	path->nodes[0] = eb = alloc_dummy_extent_buffer(NULL, 4096);
 	if (!eb) {
 		test_msg("Could not allocate dummy buffer\n");
 		ret = -ENOMEM;