Ignore kernel memory settings

This is somewhat radical approach to deal with kernel memory.

Per-cgroup kernel memory limiting was always problematic. A few
examples:

 - older kernels had bugs and were even oopsing sometimes (best example
   is RHEL7 kernel);
 - kernel is unable to reclaim the kernel memory so once the limit is
   hit a cgroup is toasted;
 - some kernel memory allocations don't allow failing.

In addition to that,

 - users don't have a clue about how to set kernel memory limits
   (as the concept is much more complicated than e.g. [user] memory);
 - different kernels might have different kernel memory usage,
   which is sort of unexpected;
 - cgroup v2 do not have a [dedicated] kmem limit knob, and thus
   runc silently ignores kernel memory limits for v2;
 - kernel v5.4 made cgroup v1 kmem.limit obsoleted (see
   https://github.com/torvalds/linux/commit/0158115f702b).

In view of all this, and as the runtime-spec lists memory.kernel
and memory.kernelTCP as OPTIONAL, let's ignore kernel memory
limits (for cgroup v1, same as we're already doing for v2).

This should result in less bugs and better user experience.

The only bad side effect from it might be that stat can show kernel
memory usage as 0 (since the accounting is not enabled).

[v2: add a warning in specconv that limits are ignored]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/specconv/spec_linux.go

@@ -21,6 +21,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/seccomp"
 	libcontainerUtils "github.com/opencontainers/runc/libcontainer/utils"
 	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/sirupsen/logrus"
 
 	"golang.org/x/sys/unix"
 )
@@ -510,11 +511,8 @@ func CreateCgroupConfig(opts *CreateOpts, defaultDevs []*devices.Device) (*confi
 				if r.Memory.Swap != nil {
 					c.Resources.MemorySwap = *r.Memory.Swap
 				}
-				if r.Memory.Kernel != nil {
-					c.Resources.KernelMemory = *r.Memory.Kernel
-				}
-				if r.Memory.KernelTCP != nil {
-					c.Resources.KernelMemoryTCP = *r.Memory.KernelTCP
+				if r.Memory.Kernel != nil || r.Memory.KernelTCP != nil {
+					logrus.Warn("Kernel memory settings are ignored and will be removed")
 				}
 				if r.Memory.Swappiness != nil {
 					c.Resources.MemorySwappiness = r.Memory.Swappiness