Update nbits_set in brin_bloom_union

Update nbits_set in brin_bloom_union

Properly update the number of bits set in the bitmap after merging the
filters in brin_bloom_union.

This is mostly harmless, as the counter is used only in the output
function, which means pageinspect may show incorrect information about
the BRIN summary. The counter does not affect correctness.

Discovered while adding a regression test comparing indexes built with
and without parallelism. The parallel index builds exercise the union
procedure when merging results from workers, which is otherwise very
hard to do in a test. Which is why this went unnoticed until now.

Backpatch through 14, where the BRIN bloom opclasses were introduced.

Backpatch-through: 14
Discussion: https://postgr.es/m/1df00a66-db5a-4e66-809a-99b386a06d86%40enterprisedb.com