CTF containers use the ctf_dtoldid field as a threshold type index which
indicates the range of type indices which have been committed to the
container by ctf_update(). However, the top bit of the dtd_type field is
not part of the type index; rather, it is a flag used to indicate that the
corresponding CTF container is a parent. This is why the maximum CTF type
index is 2^15 - 1 rather than 2^16 - 1. Therefore, this flag must be masked
off (using the CTF_TYPE_TO_INDEX macro) when comparing a type index with the
ctf_dtoldid field of a container.

This bug was causing libctf to erroneously free committed type definitions
in ctf_discard(). libdtrace holds some references to such types, resulting
in a use-after-free.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division