libperf: Warn when exceeding MAX_NR_CPUS in cpumap

Display a warning when attempting to profile more than MAX_NR_CPU CPUs.
This patch should not change any behavior.

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/20190827214352.94272-8-meyerk@stormcage.eag.rdlabs.hpecorp.net


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>