net/dccp/timer.c: use 'u64' instead of 's64' to avoid compiler's warning

'dccp_timestamp_seed' is initialized once by ktime_get_real() in
dccp_timestamping_init(). It is always less than ktime_get_real()
in dccp_timestamp().

Then, ktime_us_delta() in dccp_timestamp() will always return positive
number. So can use manual type cast to let compiler and do_div() know
about it to avoid warning.

The related warning (with allmodconfig under unicore32):

    CC [M]  net/dccp/timer.o
  net/dccp/timer.c: In function ‘dccp_timestamp’:
  net/dccp/timer.c:285: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>