pkg/csource: minor fixes
pkg/csource: minor fixes

1. Print errno with %u instead of %d
2. Avoid unused var warning for syz_emit_ethernet
   when tracing is enabled.

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