selftests/bpf: Fix ringbuf test fetching map FD

Seems like 4d1b6298 ("selftests/bpf: Convert few tests to light skeleton.")
and 704e2beb ("selftests/bpf: Test ringbuf mmap read-only and read-write
restrictions") were done independently on bpf and bpf-next trees and are in
conflict with each other, despite a clean merge. Fix fetching of ringbuf's
map_fd to use light skeleton properly.

Fixes: 704e2beb ("selftests/bpf: Test ringbuf mmap read-only and read-write restrictions")
Fixes: 4d1b6298 ("selftests/bpf: Convert few tests to light skeleton.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210618002824.2081922-1-andrii@kernel.org

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

@@ -100,7 +100,7 @@ void test_ringbuf(void)
 	if (CHECK(err != 0, "skel_load", "skeleton load failed\n"))
 		goto cleanup;
 
-	rb_fd = bpf_map__fd(skel->maps.ringbuf);
+	rb_fd = skel->maps.ringbuf.map_fd;
 	/* good read/write cons_pos */
 	mmap_ptr = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, rb_fd, 0);
 	ASSERT_OK_PTR(mmap_ptr, "rw_cons_pos");