proc: fixup uptime selftest

[ Upstream commit 5cc81d5c81af0dee54da9a67a3ebe4be076a13db ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>