Fix deadlock in heap_compute_xid_horizon_for_tuples().

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