wbthrottle: use posix_fadvise if available

Only adding information about data usage. This won't effect correctness.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #816 from ceph/wip-fadvise

wbthrottle: use posix_fadvise if available

Reviewed-by: Sage Weil <sage@inktank.com>