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.