Introduce devid2kname - slightly different to devid2devnm.

Introduce devid2kname - slightly different to devid2devnm.

The purpose od devid2devnm is to return a kernel name of an
md device, whether that device is a whole device or a partition,
we want the whole device.  md4, never md4p2.

In one place I was using devid2devnm where I really wanted the
partition if there was one ... and wasn't really interested in it
being an md device.
So introduce a new 'devid2kname' for that case.

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