block: introduce mp_bvec_for_each_page() for iterating over page

mp_bvec_for_each_segment() is a bit big for the iteration, so introduce
a light-weight helper for iterating over pages, then 32bytes stack
space can be saved.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/bvec.h

@@ -188,4 +188,9 @@ static inline void mp_bvec_last_segment(const struct bio_vec *bvec,
 	}
 }
 
+#define mp_bvec_for_each_page(pg, bv, i)				\
+	for (i = (bv)->bv_offset / PAGE_SIZE;				\
+		(i <= (((bv)->bv_offset + (bv)->bv_len - 1) / PAGE_SIZE)) && \
+		(pg = bvec_nth_page((bv)->bv_page, i)); i += 1)
+
 #endif /* __LINUX_BVEC_ITER_H */