bpf: arena: Reintroduce memcg accounting

bpf: arena: Reintroduce memcg accounting

When arena allocations were converted from bpf_map_alloc_pages() to
kmalloc_nolock() to support non-sleepable contexts, memcg accounting was
inadvertently lost. This commit restores proper memory accounting for
all arena-related allocations.

All arena related allocations are accounted into memcg of the process
that created bpf_arena.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102200230.25168-3-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>