selftests/bpf: Don't close(-1) in serial_test_fexit_stress()

Guard close() with extra link_fd[i] > 0 and fexit_fd[i] > 0
check to prevent close(-1).

Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240623131753.2133829-1-make24@iscas.ac.cn