[HACKERS] Should buffer of initialization fork have a BM_PERMANENT flag

An unlogged table has an initialization fork. The initialization fork does not have an BM_PERMANENT flag when get a buffer.

In checkpoint (not shutdown or end of recovery), it will not write to disk. 

after a crash recovery, the page of initialization fork will not correctly, then make the main fork not correctly too.

Here is an example for GIN index.

create unlogged table gin_test_tbl(i int4[]);

create index gin_test_idx on gin_test_tbl using gin (i);

checkpoint;

kill all the postgres process, and restart again.

vacuum gin_test_tbl;  -- crash.

It seems have same problem in BRIN, GIN, GiST and HASH index which using buffer for meta page initialize in ambuildempty function.