Bug#837964: mdadm --add fails, can not add spares (regression)

[Anthony DeRobertis]

Got the exact mdadm error message (well, with a different device):
mdadm: add new device failed for /dev/sdc3 as 8: Invalid argument

and the associated kernel messages for that one:

[12082.269147] md: sdc3 does not have a valid v1.0 superblock, not 
importing!
[12082.269155] md: md_import_device returned -22

sdc3 used to be in the array, but I removed it and did a 
--zero-superblock on it before trying to re-add it. (sdb3 had never been 
in the array before).