Fixes issues with mounting subdirectories of /mnt in container.

Tested with
mkdir /mnt/data
docker run -v /mnt/data:/mnt/data  -t ubuntu:12.10 touch /mnt/data/bar

Expected /mnt/data/bar on host.
Merge pull request #2150 from pnasrat/fix-mnt-subdir-volumes

Fixes issues with mounting subdirectories of /mnt in container.