block: move two bvec structure into bvec.h

This patch moves 'struct bio_vec' and 'struct bvec_iter'
into 'include/linux/bvec.h', then always include this header
into 'include/linux/blk_types.h'.

With this change, both 'struct bvec_iter' and bvec iterator
helpers don't depend on CONFIG_BLOCK any more, then we can
use bvec iterator to implement iterate_bvec(): lib/iov_iter.c.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>