lxc_setup_fs: Create /dev/shm folder if it doesn't exist

lxc_setup_fs: Create /dev/shm folder if it doesn't exist

When running application containers with lxc-execute, /dev is
populated only with device entries. Since /dev is a tmpfs mount in
the container environment, the /dev/shm folder not being present is not
a sufficient reason for the /dev/shm mount to fail.

Create the /dev/shm directory if not present.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>