Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.

Also switch from page_address to bvec_kmap_local for cbv to be on the
safe side and to avoid pointlessly poking into bvec internals.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20211020143812.6403-8-colyli@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>