memcg: make mem_cgroup_read_stat() unsigned

memcg: make mem_cgroup_read_stat() unsigned

mem_cgroup_read_stat() returns a page count by summing per cpu page
counters.  The summing is racy wrt.  updates, so a transient negative
sum is possible.  Callers don't want negative values:

 - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback.
   This could confuse dirty throttling.

 - oom reports and memory.stat shouldn't show confusing negative usage.

 - tree_usage() already avoids negatives.

Avoid returning negative page counts from mem_cgroup_read_stat() and
convert it to unsigned.

[[email protected]: fix old typo while we're in there]
Signed-off-by: Greg Thelen <[email protected]>
Cc: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: <[email protected]>	[4.2+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>