skip reading vmallocinfo if not needed

If kernel supports vmalloc memory accouting from /proc/meminfo
(https://patchwork.kernel.org/patch/10944111/

)
it's pointless to parse /proc/vmallocinfo since it's too heavy

Bug: 136688885
Test: manual checking right vmalloc pages from meminfo
Test: manual checking check that never invokes ReadVmallocInfo if
      kernel supports vmalloc pages
Change-Id: I725ea1f8a0c4ab07e78893fdb39661a65eac39e6
Signed-off-by: Minchan Kim <minchan@google.com>