gfs2: Don't wait for journal flush in clean_journal

Commit 588bff95 added gfs2_write_log_header() and started using it in
clean_journal(), with an additional call to log_flush_wait() at the end of
gfs2_write_log_header() which is unnecessary for clean_journal().  Move
that call out of gfs2_write_log_header() to restore the previous behavior.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>