Activate write-only optimization if bpf device opened with O_WRONLY.

dhclient opens bpf as write-only to send packets. It never reads received
packets from that descriptor, but processing them in kernel takes time.
Especially much time takes packet timestamping on systems with expensive
timecounter, such as bhyve guest, where network speed dropped in half.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.