cifs: fix dentry hash calculation for case-insensitive mounts

case-insensitive mounts shouldn't use full_name_hash(). Make sure we
use the parent dentry's d_hash routine when one is set.

Reported-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>