Illumos 5946, 5945

Illumos 5946, 5945

5946 zfs_ioc_space_snaps must check that firstsnap and lastsnap refer to snapshots
5945 zfs_ioc_send_space must ensure that fromsnap refers to a snapshot
Reviewed by: Steven Hartland <killing@multiplay.co.uk>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gordon.ross@nexenta.com>

References:
  https://www.illumos.org/issues/5946
  https://www.illumos.org/issues/5945
  https://github.com/illumos/illumos-gate/commit/24218be

Ported-by: Andriy Gapon <avg@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3552