tag2name() returns a uint16_t, so we don't need to use uint32_t for the
qid (or pqid). This reduces the size of struct pf_kstate slightly. That
in turn buys us space to add extra fields for dummynet later.

Happily these fields are not exposed to user space (there are user space
versions of them, but they can just stay uint32_t), so there's no ABI
breakage in modifying this.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")