Grow: improve error message is "--grow -n2" used on Linear arrays.

Linear arrays don't respond to setting raid-disks, only to
adding a device.

Reported-by: mulhern
Reported-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1122146
Signed-off-by: NeilBrown <neilb@suse.de>
Assemble: Only fail auto-assemble in face of mdadm.conf conflicts.

We should never auto-assemble things that conflict with mdadm.conf
However explicit assembly requests should be allowed.

Reported-by: olovopb
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1070245
Signed-off-by: NeilBrown <neilb@suse.de>