memcg: turn memcg_kmem_skip_account into a bit field

It isn't supposed to stack, so turn it into a bit-field to save 4 bytes on
the task_struct.

Also, remove the memcg_stop/resume_kmem_account helpers - it is clearer to
set/clear the flag inline.  Regarding the overwhelming comment to the
helpers, which is removed by this patch too, we already have a compact yet
accurate explanation in memcg_schedule_cache_create, no need in yet
another one.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1364,6 +1364,10 @@ struct task_struct {
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 
+#ifdef CONFIG_MEMCG_KMEM
+	unsigned memcg_kmem_skip_account:1;
+#endif
+
 	unsigned long atomic_flags; /* Flags needing atomic access. */
 
 	pid_t pid;
@@ -1679,8 +1683,7 @@ struct task_struct {
 	/* bitmask and counter of trace recursion */
 	unsigned long trace_recursion;
 #endif /* CONFIG_TRACING */
-#ifdef CONFIG_MEMCG /* memcg uses this to do batch job */
-	unsigned int memcg_kmem_skip_account;
+#ifdef CONFIG_MEMCG
 	struct memcg_oom_info {
 		struct mem_cgroup *memcg;
 		gfp_t gfp_mask;