selftests/bpf: Mute expected invalid map creation error msg

Previously, the newly introduced test case in test_map_in_map(), which
checks whether the inner map is destroyed after unsuccessful creation of
the outer map, logged the following harmless and expected error:

    libbpf: map 'mim': failed to create: Invalid argument(-22) libbpf:
    failed to load object './test_map_in_map_invalid.o'

To avoid any possible confusion, mute the logging during loading of the
prog.

Fixes: 08f71a1e39 ("selftests/bpf: Check inner map deletion")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721140941.563175-1-m@lambda.lt

tools/testing/selftests/bpf/test_maps.c

@@ -1163,6 +1163,7 @@ static void test_map_in_map(void)
 	struct bpf_map_info info = {};
 	__u32 len = sizeof(info);
 	__u32 id = 0;
+	libbpf_print_fn_t old_print_fn;
 
 	obj = bpf_object__open(MAPINMAP_PROG);
 
@@ -1250,12 +1251,16 @@ static void test_map_in_map(void)
 		goto out_map_in_map;
 	}
 
+	old_print_fn = libbpf_set_print(NULL);
+
 	err = bpf_object__load(obj);
 	if (!err) {
 		printf("Loading obj supposed to fail\n");
 		goto out_map_in_map;
 	}
 
+	libbpf_set_print(old_print_fn);
+
 	/* Iterate over all maps to check whether the internal map
 	 * ("mim.internal") has been destroyed.
 	 */