uires this error checking to avoid a crash, but it's better
to be proactive in case a future user happens to have a bug and
a user tries to diagnose it with CONFIG_DEBUG_PER_CPU_MAPS.

Reported-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Tejun Heo <tj@kernel.org>
LKML-Reference: <alpine.DEB.2.00.1102071407250.7812@chino.kir.corp.google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
§Žgל