selftests/bpf: fix signedness bug in redir_partial()

When xsend() returns -1 (error), the check 'n < sizeof(buf)' incorrectly
treats it as success due to unsigned promotion. Explicitly check for -1
first.

Fixes: a4b7193 ("selftests/bpf: Add sockmap test for redirecting partial skb data")
Signed-off-by: Fushuai Wang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Fushuai Wang

tools/testing/selftests/bpf/prog_tests/sockmap_listen.c

@@ -924,6 +924,8 @@ static void redir_partial(int family, int sotype, int sock_map, int parser_map)
 		goto close;
 
 	n = xsend(c1, buf, sizeof(buf), 0);
+	if (n == -1)
+		goto close;
 	if (n < sizeof(buf))
 		FAIL("incomplete write");