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)
From [godoc](http://godoc.org/os#MkdirAll): *If path is already a directory, MkdirAll does nothing and returns nil*.
So no need for the if.