The sysctl net/core/bpf_jit_enable does not work now due to Commit
1022a54 ("bpf, x86_64: Use bpf_jit_binary_pack_alloc").
The commit saved the jitted insns into 'rw_image' instead of 'image'
which caused bpf_jit_dump not dumping proper content.

With 'echo 2 > /proc/sys/net/core/bpf_jit_enable', run
'./test_progs -t fentry_test'. Without this patch, one of jitted
image for one particular prog is:
  flen=17 proglen=92 pass=4 image=0000000014c64883 from=test_progs pid=1807
  00000000: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
  00000010: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
  00000020: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
  00000030: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
  00000040: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
  00000050: cc cc cc cc cc cc cc cc cc cc cc cc

With this patch, the jitte image for the same prog is:
  flen=17 proglen=92 pass=4 image=00000000b90254b7 from=test_progs pid=1809
  00000000: f3 0f 1e fa 0f 1f 44 00 00 66 90 55 48 89 e5 f3
  00000010: 0f 1e fa 31 f6 48 8b 57 00 48 83 fa 07 75 2b 48
  00000020: 8b 57 10 83 fa 09 75 22 48 8b 57 08 48 81 e2 ff
  00000030: 00 00 00 48 83 fa 08 75 11 48 8b 7f 18 be 01 00
  00000040: 00 00 48 83 ff 0a 74 02 31 f6 48 bf 18 d0 14 00
  00000050: 00 c9 ff ff 48 89 77 00 31 c0 c9 c3

Fixes: 1022a54 ("bpf, x86_64: Use bpf_jit_binary_pack_alloc")
Signed-off-by: Yonghong Song <yhs@fb.com>