perf machine: Replace MAX_NR_CPUS with perf_env::nr_cpus_online

perf machine: Replace MAX_NR_CPUS with perf_env::nr_cpus_online

nr_cpus, the number of CPUs online during a record session bound by
MAX_NR_CPUS, can be used as a dynamic alternative for MAX_NR_CPUS in
__machine__synthesize_threads and machine__set_current_tid.

Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russ Anderson <russ.anderson@hpe.com>
Link: http://lore.kernel.org/lkml/20190827214352.94272-6-meyerk@stormcage.eag.rdlabs.hpecorp.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>