open_subarray: pass subarray name as explicit arg.

open_subarray: pass subarray name as explicit arg.

Rather than hiding this arg in the 'st' structure, pass it explicitly.

This is a first step to getting rid of 'subarray' from 'supertype'.

The strcpy in open_subarray should have better error checking, but it
will disappear soon so there is little point.

Signed-off-by: NeilBrown <neilb@suse.de.