selftests/cgroup: fix uninitialized variables in test_zswap.c

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 finds and warning about some uninitialized variables. Fix these
by initializing them.

[1] https://lore.kernel.org/all/[e-mail]/

Signed-off-by: John Hubbard <[e-mail]>
Reviewed-by: Roman Gushchin <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

tools/testing/selftests/cgroup/test_zswap.c

···
 {
 	int ret = KSFT_FAIL;
 	size_t control_allocation_size = MB(10);
-	char *control_allocation, *wb_group = NULL, *control_group = NULL;
+	char *control_allocation = NULL, *wb_group = NULL, *control_group = NULL;
 
 	wb_group = setup_test_group_1M(root, "per_memcg_wb_test1");
 	if (!wb_group)
···
 	struct sysinfo sys_info;
 	int ret = KSFT_FAIL;
 	int child_status;
-	char *test_group;
+	char *test_group = NULL;
 	pid_t child_pid;
 
 	/* Read sys info and compute test values accordingly */