cifs: fix CIFS_ENUMERATE_SNAPSHOTS oops

As with 618763958b, an open directory may have a NULL private_data
pointer prior to readdir. CIFS_ENUMERATE_SNAPSHOTS must check for this
before dereference.

Fixes: 834170c859 ("Enable previous version support")
Signed-off-by: David Disseldorp <ddiss@suse.de>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/ioctl.c

@@ -213,6 +213,8 @@ long cifs_ioctl(struct file *filep, unsigned int command, unsigned long arg)
 			rc = smb_mnt_get_fsinfo(xid, tcon, (void __user *)arg);
 			break;
 		case CIFS_ENUMERATE_SNAPSHOTS:
+			if (pSMBFile == NULL)
+				break;
 			if (arg == 0) {
 				rc = -EINVAL;
 				goto cifs_ioc_exit;