win32: only count subdirectories if necessary

Commit 7fb95a2a5 (win32: try to get link count for directories)
allowed stat(2) to report accurate values of st_nlink for
directories.

There are only a couple of places in busybox-w32 where these values
are required.  Disable counting of subdirectories by default and
only enable it when necessary.

Microsoft kindly provide directories to test edge cases like this:

   C:/Windows/WinSxS           (contains many subdirectories)
   C:/Windows/WinSxS/Manifests (contains many files)

Adds 84-112 bytes.

1 files changed, 15 insertions, 3 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index 79bcaa47d..75e6c3f24 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -619,13 +619,25 @@ static int mingw_is_directory(const char *path)
 }
 
 #if ENABLE_FEATURE_EXTRA_FILE_DATA
-static int count_subdirs(const char *pathname)
+/*
+ * By default we don't count subdirectories.  Counting can be enabled
+ * in specific cases by calling 'count_subdirs(NULL)' before making
+ * any calls to stat(2) or lstat(2) that require accurate values of
+ * st_nlink for directories.
+ */
+int count_subdirs(const char *pathname)
 {
         int count = 0;
-        DIR *dirp = opendir(pathname);
+        DIR *dirp;
         struct dirent *dp;
+        static int do_count = FALSE;
+
+        if (pathname == NULL) {
+                do_count = TRUE;
+                return 0;
+        }
 
-        if (dirp) {
+        if (do_count && (dirp = opendir(pathname))) {
                 while ((dp = readdir(dirp)) != NULL) {
                         if (dp->d_type == DT_DIR)
                                 count++;