MFC r254587: MFV r254421:

Illumos ZFS issues:
  3996 want a libzfs_core API to rollback to latest snapshot
MFC r259811: MFV r258373:

4168 ztest assertion failure in dbuf_undirty

4169 verbatim import causes zdb to segfa
4170 zhack leaves pool in ACTIVE state

illumos/illumos-gate@7fdd916c474ea52896c671bbe7b56ba34a1ca132
MFC r259813 + r259816: MFV r258374:

4171 clean up spa_feature_*() interfaces

4172 implement extensible_dataset feature for use by other zpool
features

illumos/illumos-gate@2acef22db7808606888f8f92715629ff3ba555b9