perf test lock_contention.sh: Skip test if not enough CPUs

Machines with less then 4 CPUs weren't consistently triggering lock
events required for the test.

Skip the test on those machines. The limit of 4 CPUs is set as it
generates around 100 lock events for a test.

Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Acked-by: Michael Petlan <mpetlan@redhat.com>
Link: https://lore.kernel.org/r/20230919150419.23193-2-vmolnaro@redhat.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>