pacific: common: Use double instead of long double to improve performance
To convert namoseconds to seconds, the precision needs to be 10,
and the precision of double is 15, which is enough to use.
On aarch64, double division uses the div instruction, while long
double uses the gcc buildin _divtf3, which has poor performance.
Therefore, use double instead of long double for better performance.
the parent pull request is https://github.com/ceph/ceph/pull/42082