include/ceph_features: note kernel versions some features appear
"X" means that the feature bit has been advertised and supported since
kernel X. It should be possible to use the kernel client with clusters
that don't support these features, but some of the combinations haven't
seen any testing in close to eight years (e.g. FLOCK or DIRLAYOUTHASH).
"X req" means that the feature bit has been advertised and required
since kernel X.