Reduce excessive dereferencing of function pointers

It is equivalent in ANSI C to write (*funcptr) () and funcptr().  These
two styles have been applied inconsistently.  After discussion, we'll
use the more verbose style for plain function pointer variables, to make
it clear that it's a variable, and the shorter style when the function
pointer is in a struct (s.func() or s->func()), because then it's clear
that it's not a plain function name, and otherwise the excessive
punctuation makes some of those invocations hard to read.

Discussion: https://www.postgresql.org/message-id/f52c16db-14ed-757d-4b48-7ef360b1631d@2ndquadrant.com

src/backend/commands/seclabel.c

@@ -122,7 +122,7 @@ ExecSecLabelStmt(SecLabelStmt *stmt)
 	}
 
 	/* Provider gets control here, may throw ERROR to veto new label. */
-	(*provider->hook) (&address, stmt->label);
+	provider->hook(&address, stmt->label);
 
 	/* Apply new label. */
 	SetSecurityLabel(&address, provider->provider_name, stmt->label);