selftests/bpf: Add overwrite mode test for BPF ring buffer

selftests/bpf: Add overwrite mode test for BPF ring buffer

Add overwrite mode test for BPF ring buffer. The test creates a BPF ring
buffer in overwrite mode, then repeatedly reserves and commits records
to check if the ring buffer works as expected both before and after
overwriting occurs.

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251018035738.4039621-3-xukuohai@huaweicloud.com