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 <Nanda_Kishore_Chinna@dell.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>