block: use memcpy_from_bvec in bio_copy_kern_endio_read

Use memcpy_from_bvec instead of open coding the logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-13-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>