Reduntant zip_size handling at delta application time
There is no need for a correct fil_space_create() flags value: it is never queried in the short-lived XtraBackup InnoDB file system instance, that is live only around delta application time. A valid flag value avoids assertion failures inside fil_space_create(), but it simply enough for it to be any correct value (e.g. 0).
Note that zip_size cannot be fully removed from incremental backup tablespace create because fix for bug 1043762 still requires it for the tablespace creation.
Thus the correct handling would be:
1) zip_size is written to the incremental backup tablespace metadata;
2) zip_size is optional during the metadata read;
3) fil_space_create is always passed 0 for tablespace flag as it's used only for file name hashing;
4) the actual tablespace create computes the tablespace flags from zip_size if it was read.