md: protect against crash upon fsync on ro array

If an fsync occurs on a read-only array, we need to send a
completion for the IO and may not increment the active IO count.
Otherwise, we hit a bug trace and can't stop the MD array anymore.

By advice of Christoph Hellwig we return success upon a flush
request but we return -EROFS for other writes.
We detect flush requests by checking if the bio has zero sectors.

This patch is suitable to any -stable kernel to which it applies.

Cc: Christoph Hellwig <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: [email protected]
Signed-off-by: Sebastian Riemer <[email protected]>
Reported-by: Ben Hutchings <[email protected]>
Acked-by: Paul Menzel <[email protected]>
Signed-off-by: NeilBrown <[email protected]>

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