block: Add bio_list_peek()

Introduce bio_list_peek(), to obtain a pointer to the first bio on the bio_list
without actually removing it from the list. This is needed when you want to
serialize based on the list being empty or not.
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

include/linux/bio.h

@@ -590,6 +590,11 @@ static inline void bio_list_merge_head(struct bio_list *bl,
 	bl->head = bl2->head;
 }
 
+static inline struct bio *bio_list_peek(struct bio_list *bl)
+{
+	return bl->head;
+}
+
 static inline struct bio *bio_list_pop(struct bio_list *bl)
 {
 	struct bio *bio = bl->head;