storage/stream: Move fill value handling to flash_sync

Move the code responsible for aligning the flash write by writing fill
values - from stream_flash_buffered_write to flash_sync. This avoids
having to correct buf_bytes/buf_written after the write and thus
simplifies error handling.

This commit also fixes an issue where the write length passed to the
callback in flash_sync includes the fill length.

Signed-off-by: Jonathan Nilsen <Jonathan.Nilsen@nordicsemi.no>