selftests/bpf: Fix unmap bug in prog_tests/xdp_metadata.c

The function close_xsk() unmap via munmap() the wrong memory pointer.
The call xsk_umem__delete(xsk->umem) have already freed xsk->umem.
Thus the call to munmap(xsk->umem, UMEM_SIZE) will have unpredictable
behavior that can lead to Segmentation fault elsewhere, as man page
explain subsequent references to these pages will generate SIGSEGV.

Fixes: e2a46d54 ("selftests/bpf: Verify xdp_metadata xdp->af_xdp path")
Reported-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/167527517464.938135.13750760520577765269.stgit@firesoul

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

@@ -121,7 +121,7 @@ static void close_xsk(struct xsk *xsk)
 		xsk_umem__delete(xsk->umem);
 	if (xsk->socket)
 		xsk_socket__delete(xsk->socket);
-	munmap(xsk->umem, UMEM_SIZE);
+	munmap(xsk->umem_area, UMEM_SIZE);
 }
 
 static void ip_csum(struct iphdr *iph)