cifs: add missing return value check for cifs_sb_tlink

Whenever a tlink is obtained by cifs_sb_tlink, we need
to check that the tlink returned is not an error.
It was missing with the last change here.

Fixes: b3edef6b ("cifs: allow dumping keys for directories too")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/ioctl.c

@@ -478,6 +478,11 @@ long cifs_ioctl(struct file *filep, unsigned int command, unsigned long arg)
 			}
 			cifs_sb = CIFS_SB(inode->i_sb);
 			tlink = cifs_sb_tlink(cifs_sb);
+			if (IS_ERR(tlink)) {
+				rc = PTR_ERR(tlink);
+				break;
+			}
+
 			tcon = tlink_tcon(tlink);
 			rc = cifs_dump_full_key(tcon, (void __user *)arg);
 			cifs_put_tlink(tlink);