md/md-llbitmap: finish reshape geometry

Commit the staged llbitmap geometry when reshape finishes.

When assembling a stopped reshape, md_run() creates the bitmap before
publishing mddev->pers. llbitmap_read_sb() can therefore only initialize
the reshape fields from the old on-disk sync size. Refresh the staged
reshape geometry again from llbitmap_load(), after mddev->pers is
available, and expand the in-memory page controls before replaying bitmap
state.

Reproduce on the old kernel by creating a RAID10 llbitmap with four
active disks and two spares, growing it to six disks, then stopping and
assembling while reshape is still running. The llbitmap chunk count was
32704 before grow, 49056 during reshape, then rolled back to 32704 after
reassemble.

The fixed kernel kept the target geometry across the same stop/reassemble
flow: 65440 chunks before grow, 98160 during reshape, and 98160 after
reassemble.

Link: https://lore.kernel.org/all/20260726185916.2223460-1-mykola@meshstor.io/
Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-18-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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