Grow: be careful of corrupt dev_roles list

Grow: be careful of corrupt dev_roles list

I've seen a case where the dev_roles list of a linear array
was corrupt.  ->max_dev was > 128 and > raid_disks, and the
extra slots were '0', not 0xFFFE or 0xFFFF.

This caused problems when a 128th device was added.

So:
 1/ make Grow_Add_device more robust so that if numbers
   look wrong, it fails-safe.

 2/ make examine_super1() report details if the dev_roles
   array is corrupt.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>