btrfs: simplify the error handling in __extent_writepage_io

Remove the has_error and saved_ret variables, and just jump to a goto
label for error handling from the only place returning an error from the
main loop.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>