perf mem2node: Improve warning if detected no memory nodes

perf mem2node: Improve warning if detected no memory nodes

Some archs (e.g. x86 and Arm64) don't enable the configuration
CONFIG_MEMORY_HOTPLUG by default, if this configuration is not enabled
when build the kernel image, the SysFS for memory nodes will be missed.
This results in perf tool has no chance to catpure the memory nodes
information, when perf tool reports the result and detects no memory
nodes, it outputs "assertion failed at util/mem2node.c:99".

The output log doesn't give out reason for the failure and users have no
clue for how to fix it.  This patch changes to use explicit way for
warning: it tells user that detected no memory nodes and suggests to
enable CONFIG_MEMORY_HOTPLUG for kernel building.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20201019003613.8399-1-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>