[NETFILTER]: xt_string: fix negation

The xt_string match is broken with ! negation.
This resolves a portion of netfilter bugzilla #497.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>