[TCPv6] MD5SIG: Ensure to reset allocation count to avoid panic.

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: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>