bpf: s390: Fix build error caused by the struct bpf_array member name changed

There is a build error that "'struct bpf_array' has no member
named 'prog'" on s390. In commit 2a36f0b9 ("bpf: Make the
bpf_prog_array_map more generic"), the member 'prog' of struct
bpf_array is replaced by 'ptrs'. So this patch fixes it.

Fixes: 2a36f0b9 ("bpf: Make the bpf_prog_array_map more generic")
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Kaixu Xia <xiakaixu@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>