Only warn on OOMKill disable if the feature is actually requested

Instead of warning the caller who is disabling OOM killer that the
feature isn't available, only warn if they are trying to **enable** OOM
killer and it can't be done.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)