rbd: use bvec_virt

Use bvec_virt instead of open coding it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20210804095634.460779-8-hch@lst.de


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