cifs: fix CIFS_IOC_GET_MNT_INFO oops

An open directory may have a NULL private_data pointer prior to readdir.

Fixes: 0de1f4c6 ("Add way to query server fs info for smb3")
Cc: stable@vger.kernel.org
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/ioctl.c

@@ -209,6 +209,8 @@ long cifs_ioctl(struct file *filep, unsigned int command, unsigned long arg)
 				rc = -EOPNOTSUPP;
 			break;
 		case CIFS_IOC_GET_MNT_INFO:
+			if (pSMBFile == NULL)
+				break;
 			tcon = tlink_tcon(pSMBFile->tlink);
 			rc = smb_mnt_get_fsinfo(xid, tcon, (void __user *)arg);
 			break;