tcp: avoid bogus gcc-7 array-bounds warning

When using CONFIG_UBSAN_SANITIZE_ALL, the TCP code produces a
false-positive warning:

net/ipv4/tcp_output.c: In function 'tcp_connect':
net/ipv4/tcp_output.c:2207:40: error: array subscript is below array bounds [-Werror=array-bounds]
   tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
                                        ^~
net/ipv4/tcp_output.c:2207:40: error: array subscript is below array bounds [-Werror=array-bounds]
   tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

I have opened a gcc bug for this, but distros have already shipped
compilers with this problem, and it's not clear yet whether there is
a way for gcc to avoid the warning. As the problem is related to the
bitfield access, this introduces a temporary variable to store the old
enum value.

I did not notice this warning earlier, since UBSAN is disabled when
building with COMPILE_TEST, and that was always turned on in both
allmodconfig and randconfig tests.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81601Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2202,9 +2202,10 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
 static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
 {
 	const u32 now = tcp_jiffies32;
+	enum tcp_chrono old = tp->chrono_type;
 
-	if (tp->chrono_type > TCP_CHRONO_UNSPEC)
-		tp->chrono_stat[tp->chrono_type - 1] += now - tp->chrono_start;
+	if (old > TCP_CHRONO_UNSPEC)
+		tp->chrono_stat[old - 1] += now - tp->chrono_start;
 	tp->chrono_start = now;
 	tp->chrono_type = new;
 }