[NETFILTER]: nf_conntrack: fix incorrect memset() size in FTP helper

This memset() is executing with a bad size. According to Yasuyuki Kozakai,
this memset() can be deleted, as 'ftp' is declared in global area.

Signed-off-by: Samir Bellabes <sbellabes@mandriva.com>
Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>