fs/proc: Respect boottime inside time namespace for /proc/uptime

Make sure that /proc/uptime is adjusted to the tasks time namespace.

Co-developed-by: Andrei Vagin <[e-mail]>
Signed-off-by: Andrei Vagin <[e-mail]>
Signed-off-by: Dmitry Safonov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

fs/proc/uptime.c

···
 #include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <linux/time.h>
+#include <linux/time_namespace.h>
 #include <linux/kernel_stat.h>
 
 static int uptime_proc_show(struct seq_file *m, void *v)
···
 		nsec += (__force u64) kcpustat_cpu(i).cpustat[CPUTIME_IDLE];
 
 	ktime_get_boottime_ts64(&uptime);
+	timens_add_boottime(&uptime);
+
 	idle.tv_sec = div_u64_rem(nsec, NSEC_PER_SEC, &rem);
 	idle.tv_nsec = rem;
 	seq_printf(m, "%lu.%02lu %lu.%02lu\n",