selftests/cgroup: fix clang warnings: uninitialized fd variable

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 warns about fd being used uninitialized, in
test_memcg_reclaim()'s error handling path.

Fix this by initializing fd to -1.

[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>