When commit a5a2ef07
implemented asynchronous doublewrite, the writes via
the doublewrite buffer started to be counted incorrectly,
without multiplying them by innodb_page_size.

srv_export_innodb_status(): Correctly count the
Innodb_data_written.

buf_dblwr_t: Remove submitted(), because it is close to written()
and only Innodb_data_written was interested in it. According to
its name, it should count completed and not submitted writes.

Tested by: Axel Schwenke