[CIFS] Allow null nd (as nfs server uses) on create

While creating a file on a server which supports unix extensions
such as Samba, if a file is being created which does not supply
nameidata (i.e. nd is null), cifs client can oops when calling
cifs_posix_open.

Signed-off-by: Shirish Pargaonkar <shirishp@us.ibm.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>