examples/service_cores: fix lcore count check

examples/service_cores: fix lcore count check

The example has various profiles to run services on specified
number of lcores. Due to incorrect boundary condition, service
can be dispatched to a core that does not exist. This puts main
core into endless wait.

Max available number of service cores is all detected lcores
excluding main core.

Fixes: 7f6ee6aee717 ("examples/service_cores: check cores before run")
Cc: stable@dpdk.org
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>