aoe: use bvec_kmap_local in bvcpy

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

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220303111905.321089-3-hch@lst.de


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