iflib updates and fixes:

• reset gen on down
• initialize admin task statically
• drain mp_ring on down
• don't drop context lock on stop
• reset error stats on down
• fix typo in min_latency sysctl
• return ENOBUFS from if_transmit if the driver isn't running or the link is down

Submitted by: mmacy@nextbsd.org
Reviewed by: shurd
MFC after: 2 days
Sponsored by: Isilon and Limelight Networks
Differential Revision: https://reviews.freebsd.org/D8558