md: handle read-only member devices better.

1/ If an array has any read-only devices when it is started,
   the array itself must be read-only
2/ A read-only device cannot be added to an array after it is
   started.
3/ Setting an array to read-write should not succeed
   if any member devices are read-only

Reported-and-Tested-by: Nanda Kishore Chinnaram <[email protected]>
Signed-off-by: NeilBrown <[email protected]>
Signed-off-by: Shaohua Li <[email protected]>

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