Trying to create a directory in a file should return ENOTDIR not EPERM (emscripten-core#23025)

In linux, `mkdir("some_file/dir")` returns `ENOTDIR` but Emscripten
returns `EPERM`. This fixes it.

Author: hoodmane

src/library_fs.js

@@ -372,6 +372,9 @@ FS.staticInit();
       return 0;
     },
     mayCreate(dir, name) {
+      if (!FS.isDir(dir.mode)) {
+        return {{{ cDefs.ENOTDIR }}};
+      }
       try {
         var node = FS.lookupNode(dir, name);
         return {{{ cDefs.EEXIST }}};

test/fs/test_enotdir.c

@@ -0,0 +1,17 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+
+int main() {
+  {
+    int src_fd = open("file", O_CREAT | O_WRONLY, 0777);
+    close(src_fd);
+  }
+  {
+    int target_fd = mkdir("file/blah", 0777);
+    printf("target_fd: %d, errno: %d %s\n", target_fd, errno, strerror(errno));
+  }
+}

test/fs/test_enotdir.out

@@ -0,0 +1 @@
+target_fd: -1, errno: 54 Not a directory

test/test_core.py

@@ -5852,6 +5852,10 @@ def test_fs_write(self):
   def test_fs_emptyPath(self):
     self.do_run_in_out_file_test('fs/test_emptyPath.c')
 
+  @also_with_noderawfs
+  def test_fs_enotdir(self):
+    self.do_run_in_out_file_test('fs/test_enotdir.c')
+
   @also_with_noderawfs
   def test_fs_append(self):
     self.do_runf('fs/test_append.c', 'success')

test/wasmfs/wasmfs_mkdir.c

@@ -81,13 +81,7 @@ int main() {
   // Try to make a directory with a path component that is not a directory.
   errno = 0;
   mkdir("/dev/stdout/fake-directory", 0777);
-  // TODO: This may have to change when access modes are implemented, depending
-  // on if we check access mode before file type.
-#ifdef WASMFS
   assert(errno == ENOTDIR);
-#else
-  assert(errno == EACCES);
-#endif
 
   // Try to make a directory with a path component that does not exist.
   errno = 0;