md/raid5: always convert llbitmap bits for discard

md/raid5: always convert llbitmap bits for discard

llbitmap discard is useful even when no underlying member device supports
it. The discard still converts the llbitmap range to unwritten, so later
reads and recovery do not rely on stale parity for that range.

Let llbitmap discard bypass the raid5 lower discard support check. If lower
discard is not safe or not supported, complete the accounted clone after
md_account_bio() so the llbitmap conversion callbacks run without member
discard bios.

Link: https://patch.msgid.link/20260605072639.2434847-4-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>