os/FileStore: use fdatasync(2) instead of sync_file_range(2)

This fixes data corruption on XFS.  Backported from
ffade3c85dfffa13a16edd9630a52d99eb8a413d.

Fixes: #4976
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>