selftests/bpf: Add bpf_lookup_and_delete_elem tests

Add bpf selftests and extend existing ones for a new function
bpf_lookup_and_delete_elem() for (percpu) hash and (percpu) LRU hash map
types.
In test_lru_map and test_maps we add an element, lookup_and_delete it,
then check whether it's deleted.
The newly added lookup_and_delete prog tests practically do the same
thing but additionally use a BPF program to change the value of the
element for LRU maps.

Signed-off-by: Denis Salopek <denis.salopek@sartura.hr>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/d30d3e0060c1f750e133579623cf1c60ff58f3d9.1620763117.git.denis.salopek@sartura.hr

tools/testing/selftests/bpf/test_lru_map.c

@@ -231,6 +231,14 @@ static void test_lru_sanity0(int map_type, int map_flags)
 	assert(bpf_map_lookup_elem(lru_map_fd, &key, value) == -1 &&
 	       errno == ENOENT);
 
+	/* lookup elem key=1 and delete it, then check it doesn't exist */
+	key = 1;
+	assert(!bpf_map_lookup_and_delete_elem(lru_map_fd, &key, &value));
+	assert(value[0] == 1234);
+
+	/* remove the same element from the expected map */
+	assert(!bpf_map_delete_elem(expected_map_fd, &key));
+
 	assert(map_equal(lru_map_fd, expected_map_fd));
 
 	close(expected_map_fd);