selftests: ip_local_port_range: use XFAIL instead of SKIP

SCTP does not support IP_LOCAL_PORT_RANGE and we know it,
so use XFAIL instead of SKIP.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>