libio: Update number of written bytes in dprintf implementation

The __printf_buffer_flush_dprintf function needs to record that
the buffer has been written before reusing it.  Without this
accounting, dprintf always returns zero.

Fixes commit 8ece45e4
("libio: Convert __vdprintf_internal to buffers").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>