os/FileStore: disable fadvise on XFS

fadvise(DONTNEED) on XFS can break writeback ordering and zeroing; see

      http://oss.sgi.com/archives/xfs/2013-06/msg00066.html

If we detect XFS, turn this option off.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>