Fix domain_mmap_low() to be a proper tunable.

policy/modules/kernel/domain.if

@@ -1434,14 +1434,13 @@ interface(`domain_entry_file_spec_domtrans',`
 interface(`domain_mmap_low',`
 	gen_require(`
 		attribute mmap_low_domain_type;
-		bool mmap_low_allowed;
 	')
 
 	typeattribute $1 mmap_low_domain_type;
 
-	if ( mmap_low_allowed ) {
+	tunable_policy(`mmap_low_allowed',`
 		allow $1 self:memprotect mmap_zero;
-	}
+	')
 ')
 
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