tools/syz-execprog: restore the previous tun disabling logic
tools/syz-execprog: restore the previous tun disabling logic

This probably can break some things.
I feel that tun setup can affect other things even if
syz_emit_ethernet/syz_extract_tcp_res are not used.
So it can make sense to setup tun even if they are not used.
But let's be more careful.

google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller