fix growing /dev/shm mounting
fix growing /dev/shm mounting

/dev/shm is a symlink to /run/shm, making mountpoint -q unhappy, so it would
constantly mount again.

Amit-PivotalLabs/concourse,zachgersh/concourse,sedouard/concourse,jbayer/concourse,Amit-PivotalLabs/concourse,squeedee/concourse,CloudCredo/concourse,beni55/concourse,Amit-PivotalLabs/concourse,cjcjameson/concourse,beni55/concourse,concourse/concourse,CloudCredo/concourse,concourse/concourse,holgero/concourse,zachgersh/concourse,CloudCredo/concourse,concourse/concourse,zachgersh/concourse,cjcjameson/concourse,longnguyen11288/concourse,sedouard/concourse,squeedee/concourse,concourse/concourse,longnguyen11288/concourse,longnguyen11288/concourse,concourse/concourse,beni55/concourse,sedouard/concourse,squeedee/concourse,holgero/concourse,jbayer/concourse,holgero/concourse,cjcjameson/concourse,jbayer/concourse