bpf: Do not alloc arena on unsupported arches

bpf: Do not alloc arena on unsupported arches

Do not allocate BPF arena on arches that do not support it, instead
return EOPNOTSUPP. This is useful to prevent bugs such as soft lockups
while trying to free the arena which we have witnessed on ppc64le [1].

[1] https://lore.kernel.org/bpf/4afdcb50-13f2-4772-8db1-3fd02bd985b3@redhat.com/

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/r/20241115082548.74972-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>