md/md-llbitmap: remap checkpointed bits as reshape progresses

Merge checkpointed old llbitmap state forward as reshape_position advances
and record the checkpoint remap through reshape_mark().

Normal write accounting can run while the reshape thread checkpoints a new
reshape position. llbitmap_reshape_mark() reads old state bytes, merges them
into destination bits, and writes the result back. If llbitmap_start_write()
or llbitmap_start_discard() updates the same state bytes at the same time,
the two read/modify/write paths can overwrite each other and lose the state
from one side.

Serialize only this state-byte race with a rwlock. Normal I/O takes the read
side around llbitmap_state_machine(), after page active references are raised,
so concurrent normal I/O updates still run in parallel. Reshape checkpointing
takes the write side only while merging the checkpointed range, avoiding page
suspension and avoiding a sleeping mutex in the I/O accounting path.

Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-22-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

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