Modify gethrestime to use current_kernel_time()

This allows us to get nanosecond resolution. It also means
we use the same time source as utimensat(now) etc.

Should fix issue #255. 

Note I have only tested on kernel 3.2. From a quick glance this should be okay for other kernel versions.

Also note that there is a discrepancy between the time returned from current_kernel_time() and from 'strace -ttt' for the syscall. I haven't investigated why.