net: filter: Use kcalloc/kmalloc_array to allocate arrays

Use kcalloc/kmalloc_array to make it clear we're allocating arrays. No
integer overflow can actually happen here, since len/flen is guaranteed
to be less than BPF_MAXINSNS (4096). However, this changed makes sure
we're not going to get one if BPF_MAXINSNS were ever increased.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>