selftests/bpf: Don't exit on failed bpf_testmod unload

Fix bug in handling bpf_testmod unloading that will cause test_progs exiting
prematurely if bpf_testmod unloading failed. This is especially problematic
when running a subset of test_progs that doesn't require root permissions and
doesn't rely on bpf_testmod, yet will fail immediately due to exit(1) in
unload_bpf_testmod().

Fixes: 9f7fa22 ("selftests/bpf: Add bpf_testmod kernel module for testing")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210126065019.1268027-1-andrii@kernel.org

tools/testing/selftests/bpf/test_progs.c

@@ -390,7 +390,7 @@ static void unload_bpf_testmod(void)
 			return;
 		}
 		fprintf(env.stderr, "Failed to unload bpf_testmod.ko from kernel: %d\n", -errno);
-		exit(1);
+		return;
 	}
 	if (env.verbosity > VERBOSE_NONE)
 		fprintf(stdout, "Successfully unloaded bpf_testmod.ko.\n");