posix_clocks: Use get_itimerspec64() and put_itimerspec64()

Usage of these apis and their compat versions makes
the syscalls: timer_settime and timer_gettime and their
compat implementations simpler.

This patch also serves as a preparatory patch for changing
syscalls to use new time_t data types to support the
y2038 effort by isolating the processing of user pointers
through these apis.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>