fssconfig -c not working

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

On a 6.0_BETA/amd64 machine, fssconfig -c doesn't work (it does apparently 
nothing) while it works fine without the -c.
My C foo is rather limited, i don't find any obvious errors in ffsconfig.c.
Diff'ing the two ktrace outputs yields different arguments being passed to the 
ioctl:

> -fssconfig CALL  ioctl(3,FSSIOCSET,0x7f7fffffdbe0)
> +fssconfig CALL  ioctl(3,FSSIOCSET,0x7f7fffffdbf0)
>  fssconfig GIO   fd 3 wrote 24 bytes
> -M^?\M^?\^?\^?\0\0\0\M-W\M^?\M^?\^?\^?\0\0\0\0\0\0\^?\^?\0\0"
> +-~\M^?\M^?\^?\^?\0\0\^P\M-W\M^?\M^?\^?\^?\0\0\0\0\0\0\^B\0\0\0"

May the problem be that the fss structure is not zero-filled in config()?