vendor: github.com/moby/sys/userns v0.2.1

userns: fix user-namespace detection on OpenVZ, where namespace inode
numbers are virtualized.

full diff: https://github.com/moby/sys/userns/compare/v0.2.0...v0.2.1

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>