netfilter: xt_TCPOPTSTRIP: signed tcphoff for ipv6_skip_exthdr() retval

if tcphoff remains unsigned, a negative ipv6_skip_exthdr() return value will
go unnoticed,

Signed-off-by: Roel Kluin <[email protected]>
Signed-off-by: Patrick McHardy <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
1 file changed