Clock: Using clock_gettime instead of gettimeofday in linux os.

The precision of utime_t is nsec. In func ceph_clock_now, it use
gettimeofday to set utime_t. But the precision of gettimeofday is
microseconds. In linux, the precision of clock_gettime is nsec.
So using clock_gettime install gettimeofday in order to get higer
precision.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #2589 from majianpeng/bug-fix

Clock: Using clock_gettime instead of gettimeofday in linux os.

Reviewed-by: Sage Weil <sage@redhat.com>