md/raid10: reset read_slot when reusing r10bio for discard

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.

Fixes: d30588b2731f ("md/raid10: improve raid10 discard request")
Signed-off-by: Chen Cheng <chencheng@fnnas.com>
Reviewed-by: Xiao Ni <xiao@kernel.org>
Link: https://patch.msgid.link/20260515093019.3436882-1-chencheng@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>