Fix the following warnings triggered by 51570a5a ("A Sample of
using socket cookie and uid for traffic monitoring"):

  In file included from /home/foo/net-next/samples/bpf/cookie_uid_helper_example.c:54:0:
  /home/foo/net-next/samples/bpf/cookie_uid_helper_example.c: In function 'prog_load':
  /home/foo/net-next/samples/bpf/cookie_uid_helper_example.c:119:27: warning: overflow in implicit constant conversion [-Woverflow]
     -32 + offsetof(struct stats, uid)),
                           ^
  /home/foo/net-next/samples/bpf/libbpf.h:135:12: note: in definition of macro 'BPF_STX_MEM'
   .off   = OFF,     \
            ^
  /home/foo/net-next/samples/bpf/cookie_uid_helper_example.c:121:27: warning: overflow in implicit constant conversion [-Woverflow]
     -32 + offsetof(struct stats, packets), 1),
                           ^
  /home/foo/net-next/samples/bpf/libbpf.h:155:12: note: in definition of macro 'BPF_ST_MEM'
   .off   = OFF,     \
            ^
  /home/foo/net-next/samples/bpf/cookie_uid_helper_example.c:129:27: warning: overflow in implicit constant conversion [-Woverflow]
     -32 + offsetof(struct stats, bytes)),
                           ^
  /home/foo/net-next/samples/bpf/libbpf.h:135:12: note: in definition of macro 'BPF_STX_MEM'
   .off   = OFF,     \
            ^
  HOSTLD  /home/foo/net-next/samples/bpf/per_socket_stats_example

Fixes: 51570a5a ("A Sample of using socket cookie and uid for traffic monitoring")
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>