After clearing all passwords for IPv6 peers, we need to 
set allocation count to zero as well as we free the storage.
Otherwise, we panic when a user trys to (re)add a password.

Discovered and fixed by MIYAJIMA Mitsuharu <miyajima.mitsuharu@anchor.jp>.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>