Description of problem:
d_alloc_name(NULL, "/") is working in kernel 2.6.39 or before, because d_alloc will handle NULL parent gracefully.  It will INIT_LIST_HEAD(&dentry->d_u.d_child) also.
However, in the latest source (e.g. 2.6.41 for FC15 and 3.1.1 for FC16), d_alloc_name(NULL, "/") tries to pass parent->sb to __d_alloc while parent is NULL.  Therefore, it will introduce null pointer exception.

Version-Release number of selected component (if applicable):
FC16 kernel-3.1.1-2.fc16
FC15 kernel-2.6.41.1-1.fc15

How reproducible:
In userspace, call d_alloc_name(NULL, "/").

Steps to Reproduce:
1. Compile the program.
2. Run it.
3. Check dmesg.
  
Actual results:
[   10.593715] RIP: 0010:[<ffffffff8113b60b>]  [<ffffffff8113b60b>] d_alloc+0xf/0x51
[   10.594043] Call Trace:
[   10.594047]  [<ffffffff8113b6ae>] d_alloc_name+0x61/0x63
[   10.594052]  [<ffffffffa0204dfd>] HgfsReadSuper+0x227/0x28f [vmhgfs]

Expected results:
No warning and a dummy dentry is created.

Additional info:
Use case: vmtools vmhgfs vmhgfs-only/filesystem.c HgfsReadSuper
In case d_alloc_name does not expect NULL parent, it is nice to have d_alloc_name handling it gracefully.