fs/proc/uptime.c:uptime_proc_show(): use get_monotonic_boottime()

fs/proc/uptime.c:uptime_proc_show(): use get_monotonic_boottime()

Change uptime_proc_show() to use get_monotonic_boottime() instead of
do_posix_clock_monotonic_gettime() + monotonic_to_bootbased().

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Acked-by: John Stultz <[email protected]>
Cc: Tomas Janousek <[email protected]>
Cc: Tomas Smetana <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>