mm: do not allocate shrinker info with cgroup.memory=nokmem

There'd be no work for memcg-aware shrinkers when kernel memory is not
accounted per cgroup, so we can skip allocating per memcg shrinker data.
This saves some memory, avoids holding shrinker_mutex with O(nr_memcgs)
and saves work in shrink_slab_memcg().

Then there are SHRINKER_NONSLAB shrinkers which handle non-kernel memory
so nokmem should not disable their per-memcg behavior. Such shrinkers
(e.g. deferred_split_shrinker) still need access to per-memcg data (see
also commit 0a432dcbeb32e ("mm: shrinker: make shrinker not depend on
memcg kmem")).

The savings with this patch come on container hosts that create many
superblocks (each with own shrinker) but tracking and processing per-memcg
data is pointless with nokmem (shrink_slab_memcg() is partially guarded
with !memcg_kmem_online already).

The patch uses "boottime" predicate mem_cgroup_kmem_disabled() (not
memcg_kmem_online()) to avoid mistakenly un-MEMCG_AWARE-ing shrinkers
registered before first non-root memcg is mkdir'd.

[mkoutny@suse.com: update comment, per Qi Zheng]
Link: https://lkml.kernel.org/r/20260309-cgroup-ml-nokmem-shrinker-v2-1-3e7a7eefb6c9@suse.com
Link: https://lkml.kernel.org/r/20260225-cgroup-ml-nokmem-shrinker-v1-1-d703899bdda4@suse.com
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>