block: add a bvec_phys helper

Get callers out of poking into bvec internals a bit more.  Not a huge win
right now, but with the proposed new DMA mapping API we might end up with
a lot more of this otherwise.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20240706075228.2350978-2-hch@lst.de


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