bpf: obtain timestamps from controller via pkthdr if available

r325506 (3cf8254f1ea9) extended struct pkthdr to add packet timestamp in
mbuf(9) chain. For example, cxgbe(4) and mlx5en(4) support this feature.
Use the timestamp for bpf(4) if it is available.

Reviewed by: hselasky, kib, np
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36868