netclassid: use thread_group_leader(p) in update_classid_task()

Cleanup and preparation to simplify planned future changes.

Link: https://lkml.kernel.org/r/aXY_4NSP094-Cf-2@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Christan König <christian.koenig@amd.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>