Removing faulty devices from an array is a two stage process.
First the device is moved from being a part of the active array
to being similar to a spare device. Then it can be removed
by a request from user space.
The first step is currently not performed for read-only arrays,
so the second step can never succeed.
So allow readonly arrays to remove failed devices (which aren't
blocked).
Signed-off-by: NeilBrown <neilb@suse.de>
c89a8eee61