[LTP] mmapstress06: Convert to new API

[Li Wang]

Hi Wei,

The refactoring generally looks good, but have minor issues:

1. We can remove the strict requirement for Cgroup V2.

2. Check gloabl RAM size, in Cgroup v2, memory.max limits
   physical RAM usage. If the system only has 256MB of
   physical RAM, the process can never reach the 500MB
   memory.max limit.

3. Set mem_limit/map_size to small values e.g. 128MB/256MB
   to make the test widely run.

-- 
Regards,
Li Wang