rbd: use memzero_bvec

Use memzero_bvec instead of reimplementing it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-7-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>