block: Fix cloning of requests with a special payload

This patch avoids that removing a path controlled by the dm-mpath driver
while mkfs is running triggers the following kernel bug:

    kernel BUG at block/blk-core.c:3347!
    invalid opcode: 0000 [#1] PREEMPT SMP KASAN
    CPU: 20 PID: 24369 Comm: mkfs.ext4 Not tainted 4.18.0-rc1-dbg+ #2
    RIP: 0010:blk_end_request_all+0x68/0x70
    Call Trace:
     <IRQ>
     dm_softirq_done+0x326/0x3d0 [dm_mod]
     blk_done_softirq+0x19b/0x1e0
     __do_softirq+0x128/0x60d
     irq_exit+0x100/0x110
     smp_call_function_single_interrupt+0x90/0x330
     call_function_single_interrupt+0xf/0x20
     </IRQ>

Fixes: f9d03f96 ("block: improve handling of the magic discard payload")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -3473,6 +3473,10 @@ static void __blk_rq_prep_clone(struct request *dst, struct request *src)
 	dst->cpu = src->cpu;
 	dst->__sector = blk_rq_pos(src);
 	dst->__data_len = blk_rq_bytes(src);
+	if (src->rq_flags & RQF_SPECIAL_PAYLOAD) {
+		dst->rq_flags |= RQF_SPECIAL_PAYLOAD;
+		dst->special_vec = src->special_vec;
+	}
 	dst->nr_phys_segments = src->nr_phys_segments;
 	dst->ioprio = src->ioprio;
 	dst->extra_len = src->extra_len;