possible fsync02() xfs slowness regression on power7

[CAI Qian]

> > And this commit in 3.9-rc1:
> > 
> > a1e16c2 xfs: limit speculative prealloc size on sparse files
> > should fix the problem. Please confirm these commits are the cause
> > and the fix respectively....
Confirmed this fixed the problem. I'd like to request this to be back-ported
to stable-3.0, stable-3.4 and stable-3.8. What do you think?

Thanks,
CAI Qian