rds: store socket timestamps as ktime_t

rds is the last in-kernel user of the old do_gettimeofday()
function. Convert it over to ktime_get_real() to make it
work more like the generic socket timestamps, and to let
us kill off do_gettimeofday().

A follow-up patch will have to change the user space interface
to deal better with 32-bit tasks, which may use an incompatible
layout for 'struct timespec'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>