net: add generation of semi-random BPF filters

BPF filters are used in networking and in application sand-boxing
and even have a arch-dependant JIT compiler in the kernel, so add
a more detailed semi-random BPF generator.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
net: bpf: increase the chance that the semi-random bpf is run

The kernel filter code expects us to have a return instruction and
that won't be changed in the kernel code. So increase the chance that
we can also get past the filter precheck and run this fuzzed filter
code.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>