Rename subarray v2

Rename subarray v2

Allow the name of the array stored in the metadata to be updated.  In
some cases the metadata format may not be able to support this rename
without modifying the UUID.  In these cases the request will be blocked.
Otherwise we allow the rename to take place, even for active arrays.
This assumes that the user understands the difference between the kernel
node name, the device node symlink name, and the metadata specific name.

Anticipating further need to modify subarrays in-place, introduce the
->update_subarray() superswitch method.  A future potential use
case is setting storage pool (spare-group) identifiers.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>