acct: stop using get_seconds()

In 'struct acct', 'struct acct_v3', and 'struct taskstats' we have
a 32-bit 'ac_btime' field containing an absolute time value, which
will overflow in year 2106.

There are two possible ways to deal with it:

a) let it overflow and have user space code deal with reconstructing
   the data based on the current time, or
b) truncate the times based on the range of the u32 type.

Neither of them solves the actual problem. Pick the second
one to best document what the issue is, and have someone
fix it in a future version.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/uapi/linux/taskstats.h

@@ -112,6 +112,7 @@ struct taskstats {
 	__u32	ac_gid;			/* Group ID */
 	__u32	ac_pid;			/* Process ID */
 	__u32	ac_ppid;		/* Parent process ID */
+	/* __u32 range means times from 1970 to 2106 */
 	__u32	ac_btime;		/* Begin time [sec since 1970] */
 	__u64	ac_etime __attribute__((aligned(8)));
 					/* Elapsed time [usec] */