samples/bpf: Fix the error return code of xdp_redirect's main()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

If bpf_map_update_elem() failed, main() should return a negative error.

Fixes: 832622e6 ("xdp: sample program for new bpf_redirect helper")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210616042534.315097-1-wanghai38@huawei.com