eal/linux: make alarm not affected by system time jump

eal/linux: make alarm not affected by system time jump

Due to eal_alarm_callback() and rte_eal_alarm_set() use gettimeofday()
to get the current time, and gettimeofday() is affected by jumps.

For example, set up a rte_alarm which will be triggerd next second (
current time + 1 second) by rte_eal_alarm_set(). And the callback
function of this rte_alarm sets up another rte_alarm which will be
triggered next second (current time + 2 second).
Once we change the system time when the callback function is triggered,
it is possible that rte alarm functionalities work out of expectation.

Replace gettimeofday() with clock_gettime(CLOCK_MONOTONIC_RAW, &now)
could avoid this phenomenon.

Signed-off-by: Wen-Chi Yang <wolkayang@gmail.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>