block: add for_each_mp_bvec()

Add helper of for_each_mp_bvec() for io_uring to import fixed kernel
buffer.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250325135155.935398-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

include/linux/bvec.h

@@ -184,6 +184,12 @@ static inline void bvec_iter_advance_single(const struct bio_vec *bv,
 		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
 	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
 
+#define for_each_mp_bvec(bvl, bio_vec, iter, start)			\
+	for (iter = (start);						\
+	     (iter).bi_size &&						\
+		((bvl = mp_bvec_iter_bvec((bio_vec), (iter))), 1);	\
+	     bvec_iter_advance_single((bio_vec), &(iter), (bvl).bv_len))
+
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\
 {									\