frag6.c: rename ip6q[] to ipq6b[] and consistently use "bucket"

The hash buckets array is called ip6q. The data structure ip6q is a
description of different object, the one the array holds these days
(since r337776). To clear some of this confusion, rename the array
to ip6qb.

When iterating over all buckets or addressing them directly, we
use at least the variables i, hash, and bucket. To keep the
terminology consistent use the variable name "bucket" and always
make it an uint32_t and not sometimes an int.

No functional behaviour changes intended.

MFC after: 3 months
Sponsored by: Netflix