netfilter: xt_mangle: only check verdict part of return value

These checks assume that the caller only returns NF_DROP without
any errno embedded in the upper bits.

This is fine right now, but followup patches will start to propagate
such errors to allow kfree_skb_drop_reason() in the called functions,
those would then indicate 'errno << 8 | NF_STOLEN'.

To not break things we have to mask those parts out.

Signed-off-by: Florian Westphal <fw@strlen.de>