CVE request: kernel: nfsd4: fix null dereference creating nfsv4 callback client

[Eugene Teo <eugeneteo () kernel sg>]

Quoting from upstream patch:
"On setting up the callback to the client, we attempt to use the same
authentication flavor the client did.  We find an rpc cred to use by 
calling rpcauth_lookup_credcache(), which assumes that the given 
authentication flavor has a credentials cache.  However, this is not 
required to be true--in particular, auth_null does not use one. Instead, 
we should call the auth's lookup_cred() method.

Without this, a client attempting to mount using nfsv4 and auth_null 
triggers a null dereference."

The code was introduced in upstream commit 3cef9ab2 (v2.6.31-rc1), fixed 
in 886e3b7f (v2.6.32-rc1), and was later replaced by 80fc015b in the 
same version.

References:
http://article.gmane.org/gmane.linux.nfs/26513
https://bugzilla.redhat.com/show_bug.cgi?id=530269
http://git.kernel.org/linus/3cef9ab266a932899e756f7e1ea7a988a97bf3b2
http://git.kernel.org/linus/886e3b7fe6054230c89ae078a09565ed183ecc73
http://git.kernel.org/linus/80fc015bdfe1f5b870c1e1ee02d78e709523fee7

Thanks, Eugene