selftests/bpf: Close fd in error path in drop_on_reuseport

selftests/bpf: Close fd in error path in drop_on_reuseport

In the error path when update_lookup_map() fails in drop_on_reuseport in
prog_tests/sk_lookup.c, "server1", the fd of server 1, should be closed.
This patch fixes this by using "goto close_srv1" lable instead of "detach"
to close "server1" in this case.

Fixes: 0ab5539f8584 ("selftests/bpf: Tests for BPF_SK_LOOKUP attach point")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/86aed33b4b0ea3f04497c757845cff7e8e621a2d.1720515893.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>