selftests/bpf: Adding delay in socketmap_listen to reduce flakyness

This patch adds a 1ms delay to reduce flakyness of the test.
Signed-off-by: NYucong Sun <fallentree@fb.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210819163609.2583758-1-fallentree@fb.com

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

@@ -1603,8 +1603,10 @@ static void unix_redir_to_connected(int sotype, int sock_mapfd,
 again:
 	n = read(mode == REDIR_INGRESS ? p0 : c0, &b, 1);
 	if (n < 0) {
-		if (errno == EAGAIN && retries--)
+		if (errno == EAGAIN && retries--) {
+			usleep(1000);
 			goto again;
+		}
 		FAIL_ERRNO("%s: read", log_prefix);
 	}
 	if (n == 0)
@@ -1776,8 +1778,10 @@ static void udp_redir_to_connected(int family, int sock_mapfd, int verd_mapfd,
 again:
 	n = read(mode == REDIR_INGRESS ? p0 : c0, &b, 1);
 	if (n < 0) {
-		if (errno == EAGAIN && retries--)
+		if (errno == EAGAIN && retries--) {
+			usleep(1000);
 			goto again;
+		}
 		FAIL_ERRNO("%s: read", log_prefix);
 	}
 	if (n == 0)
@@ -1869,8 +1873,10 @@ static void inet_unix_redir_to_connected(int family, int type, int sock_mapfd,
 again:
 	n = read(mode == REDIR_INGRESS ? p0 : c0, &b, 1);
 	if (n < 0) {
-		if (errno == EAGAIN && retries--)
+		if (errno == EAGAIN && retries--) {
+			usleep(1000);
 			goto again;
+		}
 		FAIL_ERRNO("%s: read", log_prefix);
 	}
 	if (n == 0)