selftests: memcg: test high limit for single entry allocation

Test the enforcement of memory.high limit for large amount of memory
allocation within a single kernel entry.  There are valid use-cases
where the application can trigger large amount of memory allocation
within a single syscall e.g.  mlock() or mmap(MAP_POPULATE).

Make sure memory.high limit enforcement works for such use-cases.

Link: https://lkml.kernel.org/r/20220211064917.2028469-4-shakeelb@google.com


Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Roman Gushchin <guro@fb.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>