mount-util: move opening of /proc/self/mountinfo into bind_remount_one_with_mountinfo()

mount-util: move opening of /proc/self/mountinfo into bind_remount_one_with_mountinfo()

Let's move things around a bit, and open /proc/self/mountinfo if needed
inside of bind_remount_one_with_mountinfo(). That way bind_remount_one()
can become a superthin inline wrapper around
bind_remount_one_with_mountinfo(). Main benefit is that we don't even
have to open /p/s/mi in case mount_setattr() actually worked for us.