riscv, bpf: Remove BPF JIT for nommu builds

The BPF JIT fails to build for kernels configured to !MMU. Without an
MMU, the BPF JIT does not make much sense, therefore this patch
disables the JIT for nommu builds.

This was reported by the kbuild test robot:

   All errors (new ones prefixed by >>):

      arch/riscv/net/bpf_jit_comp64.c: In function 'bpf_jit_alloc_exec':
   >> arch/riscv/net/bpf_jit_comp64.c:1094:47: error: 'BPF_JIT_REGION_START' undeclared (first use in this function)
       1094 |  return __vmalloc_node_range(size, PAGE_SIZE, BPF_JIT_REGION_START,
            |                                               ^~~~~~~~~~~~~~~~~~~~
      arch/riscv/net/bpf_jit_comp64.c:1094:47: note: each undeclared identifier is reported only once for each function it appears in
   >> arch/riscv/net/bpf_jit_comp64.c:1095:9: error: 'BPF_JIT_REGION_END' undeclared (first use in this function)
       1095 |         BPF_JIT_REGION_END, GFP_KERNEL,
            |         ^~~~~~~~~~~~~~~~~~
      arch/riscv/net/bpf_jit_comp64.c:1098:1: warning: control reaches end of non-void function [-Wreturn-type]
       1098 | }
            | ^

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Luke Nelson <luke.r.nels@gmail.com>
Link: https://lore.kernel.org/bpf/20200331101046.23252-1-bjorn.topel@gmail.com