kernfs,memcg: Fix CI failure for UEK4.

[imran-kn]

UEK4 (v4.1.x) kernel does not maintain separate counts for dying and active memcgroups, fix corresponding test failures with data available in UEK4.

[brenns10]

The print statements seem good enough, since it's UEK4, and it looks like this is only run for the root mem cgroup, not for every cgroup or kernfs node. Approved but let's wait until the CI pipelines pass here and on gitlab.

[brenns10]

From gitlab:

__________________ test_dump_page_cache_pages_pinning_cgroups __________________
prog = <_drgn.Program object at 0x5635a0c96be0>
    def test_dump_page_cache_pages_pinning_cgroups(prog: drgn.Program) -> None:
>       kernfs_memcg.dump_page_cache_pages_pinning_cgroups(prog, 10)
tests/test_kernfs_memcg.py:9: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
drgn_tools/kernfs_memcg.py:227: in dump_page_cache_pages_pinning_cgroups
    cgroup_state = decode_css_flags(cgrp.self.address_of_())
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
css = Object(prog, 'struct cgroup_subsys_state *', value=0xffff880ff286b800)
    def decode_css_flags(css: Object) -> str:
        """
        Get a human-readable representation of cgroup_subsys_state.flags
    
        :param css: ``struct cgroup_subsys_state *``
        """
>       CSS_DYING = css.prog_["CSS_DYING"]
E       KeyError: 'CSS_DYING'
drgn_tools/kernfs_memcg.py:44: KeyError
________________________ test_dump_memcgroup_hierarchy _________________________
prog = <_drgn.Program object at 0x5635a0c96be0>
    def test_dump_memcgroup_hierarchy(prog: drgn.Program) -> None:
>       kernfs_memcg.dump_memcgroup_hierarchy(prog)
tests/test_kernfs_memcg.py:13: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
drgn_tools/kernfs_memcg.py:82: in dump_memcgroup_hierarchy
    cgroup_state = decode_css_flags(pos)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
css = Object(prog, 'struct cgroup_subsys_state *', value=0xffff880ff23ca010)
    def decode_css_flags(css: Object) -> str:
        """
        Get a human-readable representation of cgroup_subsys_state.flags
    
        :param css: ``struct cgroup_subsys_state *``
        """
>       CSS_DYING = css.prog_["CSS_DYING"]
E       KeyError: 'CSS_DYING'
drgn_tools/kernfs_memcg.py:44: KeyError
----------------------------- Captured stdout call -----------------------------

This was with DWARF debuginfo on the ext4_dirlock-uek4 vmcore.

Turns out that specific UEK4 version is missing:

dc03f7718c0e1 ("cgroup: Prevent kill_css() from being called more than once")

Which itself is a backport of commit:

33c35aa481786 ("cgroup: Prevent kill_css() from being called more than once")

So CSS_DYING is indeed not present there.