fix issue #226

The unknown opcode typically happens if the bpf
program has an external reference which does not
get resolved. Note bcc does not even preform
relocations for maps as map_id is directly
used in bpf problem through bpf_pseudo_fd()
intrinsic.

Instead of the error:
  bpf: Failed to load program: Invalid argument
  unknown opcode 00

A little explanation is added like the below:
  HINT: The 'unknown opcode' can happen if you referencea global
  or static variable, or data in read only section.
  For example,'char *p = "hello"' will result in p referencing a
  read only section,and 'char p[] = "hello"' will have "hello"
  stored on the stack.
Signed-off-by: Yonghong Song <yhs@fb.com>