proc: fixup uptime selftest

proc: fixup uptime selftest

[ Upstream commit 124293e47461234309987a127abf7c9b32ee3faa ]

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: 70e353963592 ("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>