storage: fs: Don't attempt directory creation if it already exists

The current code attempts to handle this, but it only catches mkdir
failing with EEXIST. However if say trying to build /tmp for an
unprivileged qemu:///session, mkdir will fail with EPERM.

Rather than catch any errors, just don't attempt mkdir if the directory
already exists.

src/util/virfile.c

@@ -2289,13 +2289,14 @@ virDirCreateNoFork(const char *path,
     int ret = 0;
     struct stat st;
 
-    if ((mkdir(path, mode) < 0)
-        && !((errno == EEXIST) && (flags & VIR_DIR_CREATE_ALLOW_EXIST))) {
+    if (!((flags & VIR_DIR_CREATE_ALLOW_EXIST) && !virFileExists(path))) {
+        if (mkdir(path, mode) < 0) {
             ret = -errno;
             virReportSystemError(errno, _("failed to create directory '%s'"),
                                  path);
             goto error;
         }
+    }
 
     if (stat(path, &st) == -1) {
         ret = -errno;