Fix deadlock in heap_compute_xid_horizon_for_tuples().
We can't call code that uses syscache while we hold buffer locks
on a catalog relation. If passed such a relation, just fall back
to the general effective_io_concurrency GUC rather than trying to
look up the containing tablespace's IO concurrency setting.
We might find a better way to control prefetching in follow-up
work, but for now this is enough to avoid the deadlock introduced
by commit
558a9165e0.
Reviewed-by: Andres Freund
Diagnosed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CA%2BhUKGLCwPF0S4Mk7S8qw%2BDK0Bq65LueN9rofAA3HHSYikW-Zw%40mail.gmail.com
Discussion: https://postgr.es/m/
962831d8-c18d-180d-75fb-
8b842e3a2742%40chrullrich.net