btrfs: fix trivial -Wshadow warnings

btrfs: fix trivial -Wshadow warnings

When compiling with -Wshadow (also in 'make W=2' build) there are
several reports of shadowed variables that seem to be harmless:

- btrfs_do_encoded_write() - we can reuse 'ordered', there's no previous
			     value that would need to be preserved

- scrub_write_endio() - we need a standalone 'i' for bio iteration

- scrub_stripe() - duplicate ret2 for errors that must not overwrite 'ret'

- btrfs_subpage_set_writeback() - 'flags' is used for another irqsave lock
                                  but is not overwritten when reused for xarray
				  due to scoping, but for clarity let's rename it

- process_dir_items_leaf() - duplicate 'ret', used only for immediate checks

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>