Docker should use /var/lib/docker/tmp for large temporary files.

/tmp is often a tmpfs file system and large temporary files could cause
docker commands to fail.

Fixes #4396

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Why call this file tmpdir_linux.go and not tmpdir_unix.go?