[NETFILTER]: xt_u32: drop the actually unused variable from u32_match_it

The int ret variable is used only to trigger the BUG_ON() after
the skb_copy_bits() call, so check the call failure directly
and drop the variable.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>