bvec: add a bvec_virt helper

Add a helper to get the virtual address for a bvec.  This avoids that
all callers need to know about the page + offset representation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210804095634.460779-2-hch@lst.de


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