selftests/bpf: Close obj in error path in xdp_adjust_tail

selftests/bpf: Close obj in error path in xdp_adjust_tail

If bpf_object__load() fails in test_xdp_adjust_frags_tail_grow(), "obj"
opened before this should be closed. So use "goto out" to close it instead
of using "return" here.

Fixes: 110221081aac ("bpf: selftests: update xdp_adjust_tail selftest to include xdp frags")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/f282a1ed2d0e3fb38cceefec8e81cabb69cab260.1720615848.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>