MFV r284040: check that datasets are snapshots

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>

illumos/illumos-gate@24218bebb460e4015fac2c9f2cec1902eddbcd7b

Note that the upstream commit is modified during MFV: in the upstream
the check is done by inspecting ds_is_snapshot field while in FreeBSD
we call dsl_dataset_is_snapshot().
This is because illumos/illumos-gate@bc9014e6a81272073b9854d9f65dd59e18d18c35
(r277428 in vendor-sys/illumos) is not MFV-ed yet.

MFC after: 10 days