cifs: fix CIFS_ENUMERATE_SNAPSHOTS oops

commit 6026685de33b0db5b2b6b0e9b41b3a1a3261033c upstream.

As with 61876395, an open directory may have a NULL private_data
pointer prior to readdir. CIFS_ENUMERATE_SNAPSHOTS must check for this
before dereference.

Fixes: 834170c8 ("Enable previous version support")
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>