mm: add NR_ZSMALLOC to vmstat

mm: add NR_ZSMALLOC to vmstat

zram is very popular for some of the embedded world (e.g., TV, mobile
phones).  On those system, zsmalloc's consumed memory size is never
trivial (one of example from real product system, total memory: 800M,
zsmalloc consumed: 150M), so we have used this out of tree patch to
monitor system memory behavior via /proc/vmstat.

With zsmalloc in vmstat, it helps in tracking down system behavior due
to memory usage.

[[email protected]: zsmalloc: follow up zsmalloc vmstat]
Link: http://lkml.kernel.org/r/20160607091737.GC23435@bbox
[[email protected]: fix build with CONFIG_ZSMALLOC=m]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Minchan Kim <[email protected]>
Cc: Sangseok Lee <[email protected]>
Cc: Chanho Min <[email protected]>
Cc: Chan Gyun Jeong <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>