Mark pageinspect's disk-accessing functions as parallel restricted.

Mark pageinspect's disk-accessing functions as parallel restricted.

These functions have been marked parallel safe, but the buildfarm's
response to commit e2933a6e1 exposed the flaw in that thinking:
if you try to use them on a temporary table, and they run inside
a parallel worker, they'll fail with "cannot access temporary tables
during a parallel operation".

Fix that by marking them parallel restricted instead.  Maybe someday
we'll have a better answer and can reverse this decision.

Back-patch to v15.  To go back further, we'd have to devise variant
versions of pre-1.10 pageinspect versions.  Given the lack of field
complaints, it doesn't seem worth the trouble.  We'll just deem
this case unsupported pre-v15.  (If anyone does complain, it might
be good enough to update the markings manually in their DBs.)

Discussion: https://postgr.es/m/E1ox94a-000EHu-VH@gemulon.postgresql.org