memcg: bypass root memcg check for skmem charging

The root memcg is never associated with a socket in mem_cgroup_sk_alloc,
so there is no need to check if the given memcg is root for the skmem
charging code path.

Link: https://lkml.kernel.org/r/20250228022354.2624249-1-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>