block: Fix flush request sector restore

Make sure that a request bio is not NULL before trying to restore the
request start sector.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Fixes: 6f8fd758 ("block: Restore sector of flush requests")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20240501110907.96950-9-dlemoal@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>