cifs: fix lease break oops in xfstest generic/098

umount can race with lease break so need to check if
tcon->ses->server is still valid to send the lease
break response.

Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Fixes: 59a556aebc ("SMB3: drop reference to cfile before sending oplock break")
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/file.c

@@ -4942,9 +4942,13 @@ oplock_break_ack:
 	 * disconnected since oplock already released by the server
 	 */
 	if (!oplock_break_cancelled) {
+		/* check for server null since can race with kill_sb calling tree disconnect */
+		if (tcon->ses && tcon->ses->server) {
 			rc = tcon->ses->server->ops->oplock_response(tcon, persistent_fid,
 				volatile_fid, net_fid, cinode);
 			cifs_dbg(FYI, "Oplock release rc = %d\n", rc);
+		} else
+			pr_warn_once("lease break not sent for unmounted share\n");
 	}
 
 	cifs_done_oplock_break(cinode);