VFS: fix possible failure to unlock in nfsd4_create_file()

In the Linux kernel, the following vulnerability has been resolved:

VFS: fix possible failure to unlock in nfsd4_create_file()

atomic_create() in fs/namei.c drops the reference to the dentry

when it returns an error.

This behaviour was imported into dentry_create() so that it

will drop the reference if an error is returned from atomic_create(),

though not if vfs_create() returns an error (in the case where

->atomic_create is not supported).

The caller - nfsd4_create_file() - is made aware of this by checking

path->dentry, which will either be a counted reference to a dentry, or

an error pointer.

However the change to use start_creating()/end_creating() (which landed

shortly before the dentry_create() change landed, though was likely

developed around the same time) means that nfsd4_create_file() *needs* a

valid dentry so that it can unlock the parent.

The net result is that if NFSD exports a filesystem which uses

->atomic_create, and if a call to ->atomic_create returns an error, then

nfsd4_create_file() will pass an error pointer to end_creating()

and the parent will not be unlocked.

Fix this by changing dentry_create() to make sure path->dentry is always

a valid dentry, never an error-pointer. The actual error is already

returned a different way.

Note that if ->atomic_create() returns a different dentry (which may not

be possible in practice) we are guaranteed (because it is only ever

provided by d_spliace_alias()) that it will have the same d_parent and

so it will have the same effect when passed to end_creating().