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>