linux: make timerfd_settime(2) set expirations count to zero

linux: make timerfd_settime(2) set expirations count to zero

On Linux, read(2) from a timerfd file descriptor returns an unsigned
8-byte integer (uint64_t) containing the number of expirations
that have occurred, if the timer has already expired one or more
times since its settings were last modified using timerfd_settime(),
or since the last successful read(2).  That's to say, once we do
a read or call timerfd_settime(), timer fd's expiration count should
be zero.  Some Linux applications create timerfd and add it to epoll
with LT mode, when event comes, they do timerfd_settime instead
of read to stop event source from trigger.  On FreeBSD,
timerfd_settime(2) didn't set the count to zero, which caused high
CPU utilization.

Submitted by:	ankohuu_outlook.com (Shunchao Hu)
Differential Revision: https://reviews.freebsd.org/D28231