os/FileStore: merge filestore_replica_fadvise -> filestore_fadvise

Signed-off-by: Sage Weil <sage@redhat.com>
os/FileStore: stop disabling fadvise on XFS

This was an issue with mixing fadvise and sync_file_range, not with just
fadvise.

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