fs/proc/uptime.c: use ktime_get_boottime_ts64

fs/proc/uptime.c: use ktime_get_boottime_ts64

get_monotonic_boottime() is deprecated and uses the old timespec type.
Let's convert /proc/uptime to use ktime_get_boottime_ts64().

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Deepa Dinamani <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>