coccinelle: misc: restrict patch mode in flexible_array.cocci

Skip patches generation for structs with a single field.
Changing a zero-length array to a flexible array member in a struct
with no named members breaks the compilation. However, reporting
such cases is still valuable, e.g. commit 637464c59e
("ACPI: NFIT: Fix flexible_array.cocci warnings").

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>

scripts/coccinelle/misc/flexible_array.cocci

@@ -51,21 +51,40 @@ position p : script:python() { relevant(p) };
   };
 )
 
+@only_field depends on patch@
+identifier name, array;
+type T;
+position q;
+@@
+
+(
+  struct name {@q
+    T array[0];
+  };
+|
+  struct {@q
+    T array[0];
+  };
+)
+
 @depends on patch@
 identifier name, array;
 type T;
 position p : script:python() { relevant(p) };
+// position @q with rule "only_field" simplifies
+// handling of bitfields, arrays, etc.
+position q != only_field.q;
 @@
 
 (
-  struct name {
+  struct name {@q
     ...
     T array@p[
 -       0
     ];
   };
 |
-  struct {
+  struct {@q
     ...
     T array@p[
 -       0