golang.org/x/net/lif becomes vendor/golang_org/x/net/lif.

At git rev 9f0e377 (golang.org/cl/29893)

Updates #7177.

Change-Id: Id838fcc234e71f735bb2609073f4c2214b48a970
Reviewed-on: https://go-review.googlesource.com/29891Reviewed-by: Ian Lance Taylor <iant@golang.org>