selftests/bpf: Test bpf_skb_adjust_room on CHECKSUM_PARTIAL

When the bpf_skb_adjust_room() shrinks the skb such that its csum_start
is invalid, the skb->ip_summed should be reset from CHECKSUM_PARTIAL to
CHECKSUM_NONE.

The commit 54c3f1a8 ("bpf: pull before calling skb_postpull_rcsum()")
fixed it.

This patch adds a test to ensure the skb->ip_summed changed from
CHECKSUM_PARTIAL to CHECKSUM_NONE after bpf_skb_adjust_room().

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20221221185653.1589961-1-martin.lau@linux.dev