MFV r350896: 6585 sha512, skein, and edonr have an unenforced dependency on extensibl...

MFV r350896: 6585 sha512, skein, and edonr have an unenforced dependency on extensible dataset

illumos/illumos-gate@892586e8a147c02d7f4053cc405229a13e796928
https://github.com/illumos/illumos-gate/commit/892586e8a147c02d7f4053cc405229a13e796928

https://www.illumos.org/issues/6585
  In any pool without the extensible dataset feature flag already enabled,
  creating a dataset with dedup set to use one of the new checksums would result
  in the following panic as soon as any data was added:
  panic[cpu0]/thread=ffffff0006761c40: feature_get_refcount(spa, feature,
  &refcount) != 48 (0x30 != 0x30), file: ../../common/fs/zfs/zfeature.c line 390

  ffffff0006761830 fffffffffba8fbdd ()
  ffffff0006761890 zfs:feature_do_action+11a ()
  ffffff00067618c0 zfs:spa_feature_incr+1e ()
  ffffff0006761920 zfs:dmu_object_zapify+b7 ()
  ffffff00067619b0 zfs:dsl_dataset_activate_feature+97 ()
  ffffff0006761a20 zfs:dsl_dataset_sync+ba ()
  ffffff0006761ab0 zfs:dsl_pool_sync+153 ()
  ffffff0006761b70 zfs:spa_sync+26e ()
  ffffff0006761c20 zfs:txg_sync_thread+227 ()
  ffffff0006761c30 unix:thread_start+8 ()
  Inspection showed that feature->fi_feature was 7, which is the value of
  SPA_FEATURE_EXTENSIBLE_DATASET in the spa_feature enum.
  Testing shows that the panic can be prevented by explicitly setting extensible
  dataset as a dependency for the sha512, edonr, and skein feature flags.
  Alternatively, the new checksums code could possibly be changed to obviate the
  need for the dependency.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Richard Laager <rlaager@wiktel.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: ilovezfs <ilovezfs@icloud.com>

Note that FreeBSD does not support ednor yet.

MFC after:	2 weeks