Handle ISO 8601 leap seconds and encodings of midnight in mktime64()

Handle the following ISO 8601 features in mktime64():

 (1) Leap seconds.

     Leap seconds are indicated by the seconds parameter being the value
     60.  Handle this by treating it the same as 00 of the following
     minute.

     It has been pointed out that a minute may contain two leap seconds.
     However, pending discussion of what that looks like and how to handle
     it, I'm not going to concern myself with it.

 (2) Alternate encodings of midnight.

     Two different encodings of midnight are permitted - 00:00:00 and
     24:00:00 - the first is midnight today and the second is midnight
     tomorrow and is exactly equivalent to the first with tomorrow's date.

As it happens, we don't actually need to change mktime64() to handle either
of these - just comment them as valid parameters.

These facility will be used by the X.509 parser.  Doing it in mktime64()
makes the policy common to the whole kernel and easier to find.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
cc: John Stultz <john.stultz@linaro.org>
cc: Rudolf Polzer <rpolzer@google.com>
cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>

kernel/time/time.c

@@ -322,6 +322,13 @@ EXPORT_SYMBOL(timespec_trunc);
  * -year/100+year/400 terms, and add 10.]
  *
  * This algorithm was first published by Gauss (I think).
+ *
+ * A leap second can be indicated by calling this function with sec as
+ * 60 (allowable under ISO 8601).  The leap second is treated the same
+ * as the following second since they don't exist in UNIX time.
+ *
+ * An encoding of midnight at the end of the day as 24:00:00 - ie. midnight
+ * tomorrow - (allowable under ISO 8601) is supported.
  */
 time64_t mktime64(const unsigned int year0, const unsigned int mon0,
 		const unsigned int day, const unsigned int hour,
@@ -338,7 +345,7 @@ time64_t mktime64(const unsigned int year0, const unsigned int mon0,
 	return ((((time64_t)
 		  (year/4 - year/100 + year/400 + 367*mon/12 + day) +
 		  year*365 - 719499
-	    )*24 + hour /* now have hours */
+	    )*24 + hour /* now have hours - midnight tomorrow handled here */
 	  )*60 + min /* now have minutes */
 	)*60 + sec; /* finally seconds */
 }