clock_gettime_mono in the VFS.

[Ira Cooper]

I'm sorry that I'm coming in late on this one.  I've been a bit head down
recently.

On some platforms clock_gettime_mono has a real measurable cost, alas.  I
discovered this on Solaris/Illumos when I compiled in the timing code in
the samba 3.6 profiling framework.

I can understand why you'd want this on Linux, getting this data is pretty
painful, especially on older kernels.

But it should be a compile time option, so we don't impose a cost on
platforms where it has real cost, and on people who don't want it.

Hopefully this is something we can add. :)

Note: I expect over-time, this type of profiling will go down in
usefulness.  Linux tools to do this type of measurement are improving
quickly, and dtrace can do it fairly well on other platforms.

But today.... I totally get it, I just don't want us to regress on other
platforms.

Thanks,

-Ira