bitset: avoid pessimized code when bitset size is not constant

We have a couple optimizations for when the bitset is known to be just
one word. But with dynamically sized bitsets, it was actually more work
to determine the size than just to do the necessary computation. Now,
only use the optimization when the size is known to be constant.

Reviewed by: markj
Discussed with: jeff
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22639