perf cpumap: Increment reference count for online cpumap

perf cpumap: Increment reference count for online cpumap

Thomas Richter <tmricht@linux.ibm.com> reported a double put on the
cpumap for the placeholder core PMU:
https://lore.kernel.org/lkml/20250318095132.1502654-3-tmricht@linux.ibm.com/
Requiring the caller to get the cpumap is not how these things are
usually done, switch cpu_map__online to do the get and then fix up any
use cases where a put is needed.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20250318171914.145616-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>