bpf: Clear per_cpu pointers during bpf_prog_realloc

bpf_prog_realloc copies contents of struct bpf_prog.
The pointers have to be cleared before freeing old struct.

Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
Fixes: 700d4796 ("bpf: Optimize program stats")
Fixes: ca06f55b ("bpf: Add per-program recursion prevention mechanism")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>