correct timestamp for packets tapped by bpf

["Sepherosa Ziehau" <sepherosa@xxxxxxxxx>]

http://leaf.dragonflybsd.org/~sephe/bpf.c.diff

Above patch makes same packet tapped by different bpf devices have
same timestamp.  Obtained from FreeBSD.

Please review it.  If no objection comes, it will be committed two days later.

Best Regards,
sephe

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