meminfo - do not initialize the unused values exported by /proc/memory

Do not parse the lines of /proc/memory which refer to some values (*)
that are not used in the memory plugins.
This will save some memory when allocating 'struct proc_sysmem' and cpu cicles.

(*) Variables that have been removed:
 -  unsigned long kb_high_free;
 -  unsigned long kb_inact_target;
 -  unsigned long kb_writeback;
 -  unsigned long nr_reversemaps;
 -  unsigned long kb_mapped;
 -  unsigned long kb_pagetables;
 -  unsigned long kb_vmalloc_chunk;
 -  unsigned long kb_vmalloc_total;
 -  unsigned long kb_vmalloc_used;
 -  unsigned long kb_slab_unreclaimable;
 -  unsigned long kb_bounce;
 -  unsigned long kb_commit_limit;
 -  unsigned long kb_nfs_unstable;

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>