proc: fixup uptime selftest

syscall(3) returns -1 and sets errno on error, unlike "syscall"
instruction.

Systems which have <= 32/64 CPUs are unaffected. Test won't bounce
to all CPUs before completing if there are more of them.

Link: https://lkml.kernel.org/r/Y1bUiT7VRXlXPQa1@p183
Fixes: 1f5bd054

 ("proc: selftests: test /proc/uptime")
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>