selftests/bpf: Add sockmap test for redirecting partial skb data

Add a test case to check whether sockmap redirection works correctly
when data length returned by stream_parser is less than skb->len.

In addition, this test checks whether strp_done is called correctly.
The reason is that we returns skb->len - 1 from the stream_parser, so
the last byte in the skb will be held by strp->skb_head. Therefore,
if strp_done is not called to free strp->skb_head, we'll get a memleak
warning.

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20230804073740.194770-5-xukuohai@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>