Make get_component_size() work with named array.

Make get_component_size() work with named array.

get_component_size() still assumes that all array are
 /sys/block/md%d or /sys/block/md_d%d
and so doesn't work with e.g. /sys/block/md_foo.

This cause "mdadm --detail" to report
   Used Dev Size : unknown
and causes problems when added spares and in other circumstances.

So change it to use stat2devnm() which does the right thing with all
types of array names.

Reported-and-tested-by: Robert LeBlanc <robert@leblancnet.us>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>