md/md-llbitmap: add reshape range mapping helpers

Teach llbitmap to choose old versus new geometry during reshape and to
encode exact bitmap ranges for the active geometry. This is the mapping
groundwork for checkpoint remapping.

Range preparation now distinguishes writes from discards. Normal writes
must cover every touched bitmap chunk, while discards may only mark fully
covered chunks unwritten. Without this distinction, a discard that starts
or ends inside a chunk can make live data look unwritten after the range
has been mapped and floored.

Reproduce that with a RAID1 llbitmap using 128-sector chunks. A discard
starting halfway into chunk 8 with a 128-sector length changed clean bits
from 16352 to 16350 and unwritten bits from 0 to 2, even though no chunk
was fully discarded. With discard-specific range encoding, both counts
stay unchanged for the same test.

Range preparation also clamps the pre-map range in the same coordinate
space as the incoming IO. RAID5 receives array-sector offsets but tracks
llbitmap sync size in component sectors, so steady-state RAID5 must use
bitmap_array_sectors() before mapping and keep the existing sync-size
clamp after mapping.

Reproduce that with a 4-disk RAID5 llbitmap created --assume-clean. A
write below dev_sectors changed dirty bits from 0 to 512, but a write at
seek=2094080 left the count at 512. With the array-sector pre-map limit,
writing at seek=component_size + 65536 increased dirty bits from 512 to
1024.

Link: https://lore.kernel.org/all/20260726185916.2223460-1-mykola@meshstor.io/
Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-20-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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