block: add a bio_list_merge_init helper

This is a simple combination of bio_list_merge + bio_list_init
similar to list_splice_init.  While it only saves a single
line in a callers, it makes the move all bios from one list to
another and reinitialize the original pattern a lot more obvious
in the callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240328084147.2954434-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

include/linux/bio.h

@@ -615,6 +615,13 @@ static inline void bio_list_merge(struct bio_list *bl, struct bio_list *bl2)
 	bl->tail = bl2->tail;
 }
 
+static inline void bio_list_merge_init(struct bio_list *bl,
+		struct bio_list *bl2)
+{
+	bio_list_merge(bl, bl2);
+	bio_list_init(bl2);
+}
+
 static inline void bio_list_merge_head(struct bio_list *bl,
 				       struct bio_list *bl2)
 {