btrfs: check-integrity: simplify bio allocation in btrfsic_read_block

btrfsic_read_block does not need the btrfs_bio structure, so switch to
plain bio_alloc (that also does not fail as it's backed by a bioset).

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>