procfs: avoid 32-bit time_t in /proc/*/stat

/proc/stat shows (among lots of other things) the current boottime (i.e.
number of seconds since boot).  While a 32-bit number is sufficient for
this particular case, we want to get rid of the 'struct timespec'
suffers from a 32-bit overflow in 2038.

This changes the code to use a struct timespec64, which is known to be
safe in all cases.

Link: http://lkml.kernel.org/r/20160617201247.2292101-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/stat.c

@@ -80,19 +80,17 @@ static u64 get_iowait_time(int cpu)
 static int show_stat(struct seq_file *p, void *v)
 {
 	int i, j;
-	unsigned long jif;
 	u64 user, nice, system, idle, iowait, irq, softirq, steal;
 	u64 guest, guest_nice;
 	u64 sum = 0;
 	u64 sum_softirq = 0;
 	unsigned int per_softirq_sums[NR_SOFTIRQS] = {0};
-	struct timespec boottime;
+	struct timespec64 boottime;
 
 	user = nice = system = idle = iowait =
 		irq = softirq = steal = 0;
 	guest = guest_nice = 0;
-	getboottime(&boottime);
-	jif = boottime.tv_sec;
+	getboottime64(&boottime);
 
 	for_each_possible_cpu(i) {
 		user += kcpustat_cpu(i).cpustat[CPUTIME_USER];
@@ -163,12 +161,12 @@ static int show_stat(struct seq_file *p, void *v)
 
 	seq_printf(p,
 		"\nctxt %llu\n"
-		"btime %lu\n"
+		"btime %llu\n"
 		"processes %lu\n"
 		"procs_running %lu\n"
 		"procs_blocked %lu\n",
 		nr_context_switches(),
-		(unsigned long)jif,
+		(unsigned long long)boottime.tv_sec,
 		total_forks,
 		nr_running(),
 		nr_iowait());