bpf: Zero-fill other CPUs when BPF_F_CPU creates a per-cpu hash element

pcpu_init_value() initializes the per-cpu area of a newly created
[lru_]percpu_hash element.  The area is recycled, so when the value
comes from a BPF program (onallcpus == false) it writes the running
CPU's slot and zeroes the rest.

bpf_percpu_hash_update() passes onallcpus == true, which delegates to
pcpu_copy_value().  pcpu_copy_value() writes only the CPU named in
map_flags when BPF_F_CPU is set, so on the create path the other slots
keep the recycled element's values:

  update(k1, 0xdeadc0de, BPF_F_ALL_CPUS)  every CPU holds 0xdeadc0de
  delete(k1)                              element back on the freelist
  update(k2, 0xc0ffee, BPF_F_CPU | 0)     creates, writes CPU 0 only
  lookup(k2)                              CPU 0 0xc0ffee, rest 0xdeadc0de

Zero-fill the other CPUs on that arm too.

Fixes: c6936161fd55 ("bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for percpu_hash and lru_percpu_hash maps")
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20260924102321.2120434-2-donggeunyoo.kernel@gmail.com