xsk: account ring allocations to memcg

AF_XDP rings are allocated from setsockopt() and can be mapped into user
space.  The shared xskq_create() helper allocates the ring backing memory,
but the user-controlled and long-lived allocation is not charged as kmem
to the allocating memory cgroup.

The current implementation uses vmalloc_user(), which allocates the
backing pages with GFP_KERNEL | __GFP_ZERO.  Use the same VM_USERMAP
vmalloc path, but pass GFP_KERNEL_ACCOUNT so the ring backing pages are
attributed to memcg/kmem and can be constrained by existing cgroup memory
limits.  This keeps the existing zeroing and mmap semantics while avoiding
AF_XDP-specific optmem or RLIMIT_MEMLOCK accounting.

Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260731164623.4694-2-zihanx@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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