md/md-llbitmap: skip reading rdevs that are not in_sync

In the Linux kernel, the following vulnerability has been resolved:

md/md-llbitmap: skip reading rdevs that are not in_sync

When reading bitmap pages from member disks, the code iterates through

all rdevs and attempts to read from the first available one. However,

it only checks for raid_disk assignment and Faulty flag, missing the

In_sync flag check.

This can cause bitmap data to be read from spare disks that are still

being rebuilt and don't have valid bitmap information yet. Reading

stale or uninitialized bitmap data from such disks can lead to

incorrect dirty bit tracking, potentially causing data corruption

during recovery or normal operation.

Add the In_sync flag check to ensure bitmap pages are only read from

fully synchronized member disks that have valid bitmap data.