Fix problems found on Windows in dir_create_all

Ignore the type of error altogether. The rationale is: it doesn't matter
what was the problem if the directory is there. In the previous versions
if the directory was there already we wouldn't even attempt to create
it, so we wouldn't know about the problem neither.

Make test path length smaller in `concurrent_recursive_mkdir` test.

src/libstd/fs.rs

@@ -1777,8 +1777,7 @@ fn _create(&self, path: &Path) -> io::Result<()> {
     fn create_dir_all(&self, path: &Path) -> io::Result<()> {
         match self.inner.mkdir(path) {
             Ok(()) => return Ok(()),
-            Err(ref e)
-                if e.kind() == io::ErrorKind::AlreadyExists && path.is_dir() => return Ok(()),
+            Err(_) if path.is_dir() => return Ok(()),
             Err(ref e) if e.kind() == io::ErrorKind::NotFound => {}
             Err(e) => return Err(e),
         }
@@ -1788,7 +1787,7 @@ fn create_dir_all(&self, path: &Path) -> io::Result<()> {
         }
         match self.inner.mkdir(path) {
             Ok(()) => Ok(()),
-            Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists && path.is_dir() => Ok(()),
+            Err(_) if path.is_dir() => Ok(()),
             Err(e) => Err(e),
         }
     }
@@ -2280,7 +2279,7 @@ fn recursive_mkdir_failure() {
 
     #[test]
     fn concurrent_recursive_mkdir() {
-        for _ in 0..100 {
+        for _ in 0..50 {
             let mut dir = tmpdir().join("a");
             for _ in 0..100 {
                 dir = dir.join("a");