block: allow for_each_bvec to support zero len bvec

Block layer usually doesn't support or allow zero-length bvec. Since
commit 1bdc76a ("iov_iter: use bvec iterator to implement
iterate_bvec()"), iterate_bvec() switches to bvec iterator. However,
Al mentioned that 'Zero-length segments are not disallowed' in iov_iter.

Fixes for_each_bvec() so that it can move on after seeing one zero
length bvec.

Fixes: 1bdc76a ("iov_iter: use bvec iterator to implement iterate_bvec()")
Reported-by: syzbot <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Tested-by: Tetsuo Handa <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: <[email protected]>
Link: https://www.mail-archive.com/[email protected]/msg2262077.html
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

include/linux/bvec.h

@@ -117,11 +117,18 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
 	return true;
 }
 
+static inline void bvec_iter_skip_zero_bvec(struct bvec_iter *iter)
+{
+	iter->bi_bvec_done = 0;
+	iter->bi_idx++;
+}
+
 #define for_each_bvec(bvl, bio_vec, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
 		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
-	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
+	     (bvl).bv_len ? (void)bvec_iter_advance((bio_vec), &(iter),	\
+		     (bvl).bv_len) : bvec_iter_skip_zero_bvec(&(iter)))
 
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\