mm: show number of vmalloc pages in /proc/meminfo

Vmalloc() is getting more and more used these days (kernel stacks,
bpf and percpu allocator are new top users), and the total %
of memory consumed by vmalloc() can be pretty significant
and changes dynamically.

/proc/meminfo is the best place to display this information:
its top goal is to show top consumers of the memory.

Since the VmallocUsed field in /proc/meminfo is not in use
for quite a long time (it has been defined to 0 by the
commit a5ad88ce ("mm: get rid of 'vmalloc_info' from
/proc/meminfo")), let's reuse it for showing the actual
physical memory consumption of vmalloc().

Change-Id: Ice4fd4b94c9c156b463487aaef033a8f8c3aa821
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Patch-mainline: linux-mm @ Mon, 25 Feb 2019 12:30:37 -0800
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>