tools: only check for O_RDONLY

tools: only check for O_RDONLY

On some Android systems the lxc folders where containers are stored might be
read-only and so checking for O_RDWR, will effectively make the tools useless
on these systems, so let's dumb the check down to O_RDONLY.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>