improve encryption/compression check in ALTER TABLE IMPORT

fil_iterate() and FetchIndexRootPages::run() decrypt and decompress pages. Both eventually iterate all the pages in a table. And both can and should perform a better page validation. Like, if FSP flags say that the page should be encrypted but it's not encrypted and error must be reported. Similar should be done with page compression. https://jira.mariadb.org/browse/MDEV-18543 when landed will have a similar check which is already a little stricter. The check could be copied to the above function and if possible it should be improved further.