fil_iterate(): Invoke fil_encrypt_buf() correctly when
a ROW_FORMAT=COMPRESSED table with a physical page size of
innodb_page_size is being imported. Also, validate the page checksum
before decryption, and reduce the scope of some variables.

AbstractCallback::operator()(): Remove the parameter 'offset'.
The check for it in FetchIndexRootPages::operator() was basically
redundant and dead code since the previous refactoring.