The MLOG_FILE_WRITE_CRYPT_DATA record was completely redundant.
It can be replaced with a single MLOG_WRITE_STRING record.

To facilitate upgrade from older versions, we will retain
fil_parse_write_crypt_data().

fil_crypt_parse(): Recover fil_space_crypt_t::write_page0().

fil_space_crypt_t::write_page0(): Write everything in a single
MLOG_WRITE_STRING for easy parsing.

fil_space_crypt_t::page0_offset: Remove.