selftests/bpf: Adjust test_access_variable_array after a kernel function name change

After commit 4c3e509e

 ("sched/balancing: Rename load_balance() => sched_balance_rq()"),
the load_balance kernel function is renamed to sched_balance_rq.

This patch adjusts the fentry program in test_access_variable_array.c
to reflect this kernel function name change.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240516170140.2689430-1-martin.lau@linux.dev