Prevent pg_enable/disable_data_checksums() on standby

These functions missed a RecoveryInProgress() check, allowing them to
be called on a hot standby.  Enabling, or disabling, checksums on the
standby only would cause the cluster to get out of sync and replaying
checksum transitions to fail.

Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHg+QDfRk4-S7DMmdbXJnQ-xF=sUpMAKuh8b83ObLqYVKx5QLA@mail.gmail.com

src/backend/postmaster/datachecksum_state.c

@@ -487,6 +487,8 @@ AbsorbDataChecksumsBarrier(ProcSignalBarrierType barrier)
 Datum
 disable_data_checksums(PG_FUNCTION_ARGS)
 {
+	PreventCommandDuringRecovery("pg_disable_data_checksums()");
+
 	if (!superuser())
 		ereport(ERROR,
 				errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
@@ -507,6 +509,8 @@ enable_data_checksums(PG_FUNCTION_ARGS)
 	int			cost_delay = PG_GETARG_INT32(0);
 	int			cost_limit = PG_GETARG_INT32(1);
 
+	PreventCommandDuringRecovery("pg_enable_data_checksums()");
+
 	if (!superuser())
 		ereport(ERROR,
 				errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),