md/raid10: reset read_slot when reusing r10bio for discard

In the Linux kernel, the following vulnerability has been resolved: md/raid10: reset read_slot when reusing r10bio for discard put_all_bios() always drops devs[i].bio, but it only drops devs[i].repl_bio when r10_bio->read_slot < 0. If discard reuses an r10bio that was previously used for a read, read_slot can still be non-negative, and discard cleanup can skip bio_put() on repl_bio. Reset read_slot to -1 when preparing an r10bio for discard so the replacement bio is always released correctly.