os/FileStore: drop posix_fadvise(...DONTNEED)

On XFS this call is problematic because it directly calls the filemap
writeback without vectoring through xfs.  This can break the delicate
ordering of writeback and range zeroing; see #4976 and this thread

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

Drop this behavior for now to avoid subtle data corruption.

Signed-off-by: Sage Weil <sage@inktank.com>