Fix old_value argument retrieval in timerfd_settime parser

Fix old_value argument retrieval in timerfd_settime parser

This is done similar to timer_settime syscall parser.

* time.c (SYS_FUNC(timerfd_settime)): Retrieve old_value argument
on exiting and not on entering.  Return 0 instead of RVAL_DECODED
since the call hasn't been decoded in full on entering.