CIFS: fix use-after-free of the lease keys

The request buffers are freed right before copying the pointers.
Use the func args instead which are identical and still valid.

Simple reproducer (requires KASAN enabled) on a cifs mount:

echo foo > foo ; tail -f foo & rm foo

Cc: <stable@vger.kernel.org> # 4.20
Fixes: 179e44d4 ("smb3: add tracepoint for sending lease break responses to server")
Signed-off-by: NAurelien Aptel <aaptel@suse.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reviewed-by: NPaulo Alcantara <palcantara@suse.de>

fs/cifs/smb2pdu.c

@@ -4441,8 +4441,8 @@ SMB2_lease_break(const unsigned int xid, struct cifs_tcon *tcon,
 	rc = cifs_send_recv(xid, ses, &rqst, &resp_buf_type, flags, &rsp_iov);
 	cifs_small_buf_release(req);
 
-	please_key_low = (__u64 *)req->LeaseKey;
-	please_key_high = (__u64 *)(req->LeaseKey+8);
+	please_key_low = (__u64 *)lease_key;
+	please_key_high = (__u64 *)(lease_key+8);
 	if (rc) {
 		cifs_stats_fail_inc(tcon, SMB2_OPLOCK_BREAK_HE);
 		trace_smb3_lease_err(le32_to_cpu(lease_state), tcon->tid,