zfs: merge OpenZFS master-bf156c966

Notable upstream changes:

bf156c966 Remove unused abd_alloc_scatter_offset_chunkcnt
658fb8020 Add "compatibility" property for zpool feature sets

This update introduces a new pool property called "compatibility"
that can be used to enable a limited set of pool features on pool
creation and "stick" to it, so the "zpool upgrade" does not
accidentally enable features that are not desired. The value of
this property may then be changed later.

See zpool-features(5) for more information about the "compatibility"
pool property.

Obtained from: OpenZFS
MFC after: 2 weeks