block: introduce bio helpers for converting to multipage bvec

The following helpers are introduced for converting current users of
direct access to bvec table, and prepares for supporting multipage bvec:

	bio_pages_all()
	bio_first_bvec_all()
	bio_first_page_all()
	bio_last_bvec_all()

All are named as bio_*_all() to following bio_for_each_segment_all(),
they can only be used on bio of !bio_flagged(bio, BIO_CLONED), that means
the whole bvec table is covered.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/bio.h

@@ -300,6 +300,29 @@ static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
 		bv->bv_len = iter.bi_bvec_done;
 }
 
+static inline unsigned bio_pages_all(struct bio *bio)
+{
+	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));
+	return bio->bi_vcnt;
+}
+
+static inline struct bio_vec *bio_first_bvec_all(struct bio *bio)
+{
+	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));
+	return bio->bi_io_vec;
+}
+
+static inline struct page *bio_first_page_all(struct bio *bio)
+{
+	return bio_first_bvec_all(bio)->bv_page;
+}
+
+static inline struct bio_vec *bio_last_bvec_all(struct bio *bio)
+{
+	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));
+	return &bio->bi_io_vec[bio->bi_vcnt - 1];
+}
+
 enum bip_flags {
 	BIP_BLOCK_INTEGRITY	= 1 << 0, /* block layer owns integrity data */
 	BIP_MAPPED_INTEGRITY	= 1 << 1, /* ref tag has been remapped */