mm: memcontrol: account "kmem" consumers in cgroup2 memory controller

The original cgroup memory controller has an extension to account slab
memory (and other "kernel memory" consumers) in a separate "kmem"
counter, once the user set an explicit limit on that "kmem" pool.

However, this includes various consumers whose sizes are directly linked
to userspace activity.  Accounting them as an optional "kmem" extension
is problematic for several reasons:

1. It leaves the main memory interface with incomplete semantics. A
   user who puts their workload into a cgroup and configures a memory
   limit does not expect us to leave holes in the containment as big
   as the dentry and inode cache, or the kernel stack pages.

2. If the limit set on this random historical subgroup of consumers is
   reached, subsequent allocations will fail even when the main memory
   pool available to the cgroup is not yet exhausted and/or has
   reclaimable memory in it.

3. Calling it 'kernel memory' is misleading. The dentry and inode
   caches are no more 'kernel' (or no less 'user') memory than the
   page cache itself. Treating these consumers as different classes is
   a historical implementation detail that should not leak to users.

So, in addition to page cache, anonymous memory, and network socket
memory, account the following memory consumers per default in the
cgroup2 memory controller:

     - threadinfo
     - task_struct
     - task_delay_info
     - pid
     - cred
     - mm_struct
     - vm_area_struct and vm_region (nommu)
     - anon_vma and anon_vma_chain
     - signal_struct
     - sighand_struct
     - fs_struct
     - files_struct
     - fdtable and fdtable->full_fds_bits
     - dentry and external_name
     - inode for all filesystems.

This should give us reasonable memory isolation for most common
workloads out of the box.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>