MFV r272500:

Don't inherit flags other than DS_FLAG_CI_DATASET and DS_FLAG_INCONSISTENT
when cloning.  This prevents DS_FLAG_DEFER_DESTROY being inherited from a
clone that is marked for deferred destroy, which causes snapshots of the
clone being destroyed when getting a hold or clone.

Illumos issue:
    5150 zfs clone of a defer_destroy snapshot causes strangeness

MFC after:	1 week
MFV r272501:

Illumos issue:
    5177 remove dead code from dsl_scan.c

MFC after:	2 weeks
Minor code styling.

Suggested by:	glebius @
devfs: don't take proctree_lock unconditionally in devfs_close

MFC after:	1 week