Fix issue #1562.

Currently, bcc allows a map defined in one module and
used in the same or a different module. At tests/python directory,
test_clang.py and test_shared_table.py have such an example.
  ...
  b1 = BPF(text="""BPF_TABLE_PUBLIC("hash", int, int, table1, 10);""")
  b2 = BPF(text="""BPF_TABLE("extern", int, int, table1, 10);""")
  t = b2["table"]
  ...

With current implementation, the map key/value types are parsed
twice, resulting a type "int int" instead of "int". When type
is retrieved through `t = b2["table"]`, python decoding error
will appear.

The issue is fixed by preventing the second map key/value type
parsing if the map is `extern`. Also add `t = b2["table"]`
so the issue will be triggered if bcc clang frontend fix is not there.
Signed-off-by: Yonghong Song <yhs@fb.com>