selftests/cgroup: fix clang build failures for abs() calls

First of all, in order to build with clang at all, one must first apply
Valentin Obst's build fix for LLVM [1]. Once that is done, then when
building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...clang is pickier than gcc, about which version of abs(3) to call,
depending on the argument type:

   int abs(int j);
   long labs(long j);
   long long llabs(long long j);

...and this is causing both build failures and warnings, when running:

    make LLVM=1 -C tools/testing/selftests

Fix this by calling labs() in value_close(), because the arguments are
unambiguously "long" type.

[1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>