coccinelle: avoid matching 'errno' as a file descriptor

The `coccinelle/take-fd.cocci` transformation file attempts to rewrite

r = fd;
fd = -1;

to

r = TAKE_FD(fd);

Unfortunately, using `identifier` or `idexpression` as a metavariable
type in this case wouldn't match more complex location descriptions,
like:

x->fd = fd
fd = -1;

Using 'expression' metavariable type generates false positives,
as you can't specify scope of such expression. The only real example
from the current codebase is the global 'errno' variable, which results
in following patch generated by `spatch`:

--- src/basic/errno-util.h
+++ /tmp/cocci-output-28263-971baa-errno-util.h
@@ -15,8 +15,7 @@ static inline void _reset_errno_(int *sa

 #define UNPROTECT_ERRNO                         \
         do {                                    \
-                errno = _saved_errno_;          \
-                _saved_errno_ = -1;             \
+                errno = TAKE_FD(_saved_errno_);             \
         } while (false)

 static inline int negative_errno(void) {

Let's explicitly state that the matched expression should not equal
'errno' to avoid this. It's not particularly nice, but it should be
enough, at least for now.

coccinelle/take-fd.cocci

@@ -6,8 +6,15 @@ expression q;
 - q = -1;
 - return p;
 + return TAKE_FD(q);
+
+/* The ideal solution would use 'local idexpression' to avoid matching errno,
+ * which is a global variable. However, 'idexpression' nor 'identifier'
+ * would match, for example, "x->fd", which is considered 'expression' in
+ * the SmPL grammar
+ */
 @@
-expression p, q;
+expression p != errno;
+expression q;
 @@
 - p = q;
 - q = -1;