Do not emit WAL for unlogged BRIN indexes

Do not emit WAL for unlogged BRIN indexes

Operations on unlogged relations should not be WAL-logged. The
brin_initialize_empty_new_buffer() function didn't get the memo.

The function is only called when a concurrent update to a brin page
uses up space that we're just about to insert to, which makes it
pretty hard to hit. If you do manage to hit it, a full-page WAL record
is erroneously emitted for the unlogged index. If you then crash,
crash recovery will fail on that record with an error like this:

    FATAL:  could not create file "base/5/32819": File exists

Author: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://www.postgresql.org/message-id/CALdSSPhpZXVFnWjwEBNcySx_vXtXHwB2g99gE6rK0uRJm-3GgQ@mail.gmail.com
Backpatch-through: 14