samples/bpf: Fix wrong allocation size in xdp_router_ipv4_user

prefix_key->data allocates three bytes using alloca(), but four bytes are
actually accessed in the program.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/tencent_F9E2E81922B0C181D05B96DAE5AB0ACE6B06@qq.com