selftest/bpf: Fix error usage of ASSERT_OK in xdp_adjust_tail.c

xdp_adjust_tail.c calls ASSERT_OK() to check the return value of
bpf_prog_test_load(), but the condition is not correct. Fix it.

Fixes: 791cad02 ("bpf: selftests: Get rid of CHECK macro in xdp_adjust_tail.c")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20221011120108.782373-7-xukuohai@huaweicloud.com