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

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

[ Upstream commit 7c6090ee2a7b3315410cfc83a94c3eb057407b25 ]

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: 50d9775ce366 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>