mm/kfence: print disabling or re-enabling message

mm/kfence: print disabling or re-enabling message

By printing information, we can friendly prompt the status change
information of kfence by dmesg and record by syslog.

Also, set kfence_enabled to false only when needed.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jackie Liu <[email protected]>
Co-developed-by: Marco Elver <[email protected]>
Signed-off-by: Marco Elver <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>