There is no need to forward declare flush_write_bio(), as it only
depends on submit_one_bio().  Both of them are pretty small, just move
them to kill the forward declaration.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NQu Wenruo <wqu@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>