lib/iov_iter.c: initialize bi.bi_idx before iterating over bvec

lib/iov_iter.c: initialize bi.bi_idx before iterating over bvec

Initialize bi.bi_idx as 0 before iterating over bvec, otherwise
garbage data can be used as ->bi_idx.

Cc: Christoph Hellwig <hch@lst.de>
Reported-and-tested-by: Klara Modin <klarasmodin@gmail.com>
Fixes: e4e535bff2bc ("iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>