selftest/bpf: Ensure no module loading in bpf_setsockopt(TCP_CONGESTION)

This patch adds a test to ensure bpf_setsockopt(TCP_CONGESTION, "not_exist")
will not trigger the kernel module autoload.

Before the fix:

  [   40.535829] BUG: sleeping function called from invalid context at include/linux/sched/mm.h:274
  [...]
  [   40.552134]  tcp_ca_find_autoload.constprop.0+0xcb/0x200
  [   40.552689]  tcp_set_congestion_control+0x99/0x7b0
  [   40.553203]  do_tcp_setsockopt+0x3ed/0x2240
  [...]
  [   40.556041]  __bpf_setsockopt+0x124/0x640
Signed-off-by: Martin KaFai Lau <martin.lau@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220830231953.792412-1-martin.lau@linux.dev

tools/testing/selftests/bpf/progs/setget_sockopt.c

@@ -32,6 +32,7 @@ struct sockopt_test {
 	unsigned int flip:1;
 };
 
+static const char not_exist_cc[] = "not_exist";
 static const char cubic_cc[] = "cubic";
 static const char reno_cc[] = "reno";
 
@@ -307,6 +308,9 @@ static int bpf_test_tcp_sockopt(__u32 i, struct loop_ctx *lc)
 		const char *new_cc;
 		int new_cc_len;
 
+		if (!bpf_setsockopt(ctx, IPPROTO_TCP, TCP_CONGESTION,
+				    (void *)not_exist_cc, sizeof(not_exist_cc)))
+			return 1;
 		if (bpf_getsockopt(ctx, IPPROTO_TCP, TCP_CONGESTION, old_cc, sizeof(old_cc)))
 			return 1;
 		if (!bpf_strncmp(old_cc, sizeof(old_cc), cubic_cc)) {